Very much agree with Lee.
Another, significant difference for people coming from other RPGs (not only DnD, and not just fantasy games) is the power difference between magi and other character types.
This needs to be made clear, because, depending on what story you're running, some people may expect a traditional adventuring party to be on a roughly even footing, and be disheartened when, for example, every character save the maga proves utterly vulnerable to magic.
Comparing D&D to AM power levels of characters, I would give level ranges for each type to people coming over from D&D for each type so they would not get blindsided. I would also include aging effects since AM is a game that spans decades if not centuries.
Grogs are 1st to 3rd level in power. Those at the top of the range are rare since they have to be optimized enough to attain it before going into decline from aging (for many specialties). The things that just Grogs can handle would be what you see beginning characters in D&D deal with.
Companions are 1st to 5th level in power, though commonly towards the middle of that range. About the power level of a D&D character who has a few successful adventures under their belt. Add in that this is the power level all mundane humans generally top out at.
Magical & Mythic Companions are 1st to 10th level in power. While very powerful compared to Grogs in their area of specialty, they top out in the range of brand new to very young Magi.
Magi are in the 5th to 10th level right out of gauntlet. From there they can rapidly go up into the epic (21+) level of power. Right from the beginning, the weakest Magi can generally compete with the most powerful mundane human. Give them a few decades and they can reach levels of power which let them crush whole armies single handed.
Especially in long running Sagas you will often find activities where the Magi do not become directly involved in. OOC the Magi are just overkill power for it while IC they are busy with their research and what is the point of having minions if they do not take care of the "unimportant" stuff.
Social interaction (including group leadership) is the one area where the Magi will generally be by far the worst at. This is the area in which Companions can really shine. I am not saying to build umber diplomats. Instead look at the areas the players are interested in running stories in or which are fundamental to the Covenants stability and success. If your Covenant gets a large portion of its finances through sea trade, then having a ships captain and a merchant Companion can really help out. While a Grog could fill the role, if it is something that many game sessions might center around then having a Companion or two who can lead it is really helpful.
And if players want martial characters, they can, but they won't be the group's killing machine.
An in-game example is the shield grog, which is a kind of bodyguard for magi. While your maga may be able to fry a knight, she doesn't have eyes in the back, or may take an arrow while doing so. The shield grog is here to protect her from that.
Likewise, in any situation in which magi could be overwhelmed, martial characters can be useful, not unlike an anime where the hero fights the main boss while his pals deal with the boss underlings.
At the end of the day, each and every troupe, or even each and every player, has to figure out how to play the range of characters that befit the stories.
It is easy enough to envision how a range of unbalanced characters can work together. The magi are the lords who run the covenant and have mighty powers, but they are rarely very good at stealth and awareness, and they tend to be even worse at social skill. Not even their own crew trust them. The knight companion has natural authority, and may be the ultimate killer as long as the opposition is not supernatural. But natural leadership does not make him the best placed to collect village gossip. The grogs are commoners, that is expendable, but there are tasks they do better than anybody else; not only because they are good at it, but because magi and companions may consider it beneath them.
Actually roleplaying all these characters in practice is a different kettle of fish. The social distinctions of the 13th Century are alien to us, and therefore hard to play. We typically end up with more characters than players, so to make them interesting somebody have to play more than one. Some do not want to, and some who want to try simply forget about the secondary character. This can easily degenerate to a tactical game, where grogs are merely managed as pawns. Now, I can enjoy a tactical game, but I do not think Ars Magica has a very good ruleset for such game.
Even if the troupe is large and the group is small, there are a lot of players who want to focus entirely on their magi, and many SGs who find it difficult to design the stories to involve everybody. Even managing the mundane characters between sessions may turn out to be a chore.
Whether the martial character is the group's killing machine depends on the story. Many magi do not have the arts to get out of an emergency. I play one with painful magi, and that's one real killer. Stories can be weaved around magi who entirely depend on their martial escort. If that is what the troupe desires.
As more or less experienced players, I can think we can say a lot about what has worked and hasn't worked for us, and what me may want to achieve or avoid, but any statements about how Ars Magica is or should be played are non-sense. Everybody needs to find a way which works for them. Copying somebody else is rarely a viable solution, neither in game nor in life.
I prefer to game as a small group, so for the two dungeon crawls that I ran, I told my three players to both play a mage and a grog/companion on the adventure. The rest of the time, they'd have only a character each. The exception being when stuff was happening in the Covenant, where they could just switch characters like it was an isometric cRPG...
While each group should limit themselves to only playing the characters they are interested in, only playing Magi (with or without the other odd character thrown in) does limit you to not experiencing the extremely wide range that is possible in the system. AM is fairly unique among RPGs in that not only is it possible to play a massive range of power among characters, it is actually encouraged.
While playing a Magus is fun, I have personally found that I enjoy games in which the Magus is in situations in which their magic can not overcome issues (such as role playing out a Tribunal or interaction with another Covenant) or lesser characters take the forefront. The most fun I had was a series of games in which we ran Grogs, Apprentices, and Hermetic Cats in which not only could we not ask the Magi for help, we did not want them to find out what we were doing.
This is based on my personal experience, but I would strongly recommend playing at least a few sessions in which only one or none of the Magi are involved. AM lends itself to playing both extremely high-power/high-stakes sessions and simple low-power/low-stakes sessions within the same saga (campaign in D&D terms). Running a few sessions on the bottom end of the scale can be both refreshing and a welcome breather for the primary SG. Even better if someone else runs it, which lets the primary SG take a break and actually play a few sessions themself.
Yeah my idea was the first session or two is just apprentice, companions and grogs.
As a larger group the plan was only 1 character each but I would flip between parallel activities so different people get to play the Magi in different story lines.
I am a double history major (classical and medieval/early modern European) so will look to flesh out the society in the game both in a primer for the players and then continuing throughout the sagas. Absolutely a trap to put our 21st century perceptions over the top these older centuries.
Always amusing to hear "I wish I lived in XXXX century". Oh? really? Maybe if you were an aristocrat but let me tell you what it was actually like living then in the different classes...
Oh to heck with that, I want to live in XXXX century as a myself with my full knowledge of modern chemistry, firearms design and electrical engineering. Picking up the local language would be useful though, I don't want to have to struggle with old English and my French is horrid.
As you've said that your group is already on the larger side (6, right?) there is no need to double up, it would be really unwieldy. This was me dealing with a small party.
If your entire group is new (including storyguide) I definitely suggest not building complex full characters and going at it - I'd actually suggest using the pre-gen in the book, run a 'background' adventure for 1-3 sessions, and then have them make their own characters for the longer saga. It's an idea I like, suggested by someone earlier in this thread.
I might even go so far as to have the 'testing' adventure be the precursor to the covenant, and possibly have the players playing the former apprentices of their earlier characters. It lets them play around wiht a lot of ideas and then change things.
Part of the decisions you'll need to make is how much you want to stick to the numbers of the game, because the numbers can be VERY binding sometimes. I tend to let my players get away with things a lot.
Running a Covenant found adventure, or even a Covenant location scouting adventure (CLSA hence forth), would be a good way to introduce your players to the system. With a CLSA you can set the time to earlier than the "in game" current day, based on how old you want your Covenant to be.
A CLSA might be a good option, since if the players end up liking the location you can use it and if they don't then it is just a failed attempt. You just need to grab a host of pre-gen Grogs from online or a book (I would suggest Grogs book), give them a few enchanted and invested items to get a taste of magic, and let them scout out the location. You can do more than one location if you like. At the very least they would need something that detects magic aura and gives some indication of its strength. An item that glows, with how bright the glow is correlating to the strength of the Aura, is not a very difficult item to create.
If you wanted your Covenant to be a Summer one 50 years post founding, then you would set the date during the adventure to something like 1160. A decade or so further back would give time for the Covenant location to be built up after it was selected but before it became official.
I would recommend not playing a brand new Spring Covenant. There is a massive resource shortage in them and the feel is very different from almost any other AM game you can play. Rather than researching and enchanting, the Magi will spend most of their time fighting to find study material, vis, and funding to keep the whole place from imploding.
I am of the opposite view, fighting with these shortages and going out to do stuff is what I find interesting. Just sitting in the lab making numbers be big like it was Eve online just isn't so exciting. But this is really a party preference.