Personally I think that companions and grogs are best introduced later on regardless. As the game gets rolling and the seasons start going by, the players will start wanting their magi to stay at home. Grogs are a good second character to introduce, companions are best left until the troupe concept is a bit more solid. Rushing into creating companions early can result in companions and magi that just don't gel well - and companions that end up getting no play time.
For a magi-focused group, if they are all fresh out of apprenticeship there will still be many things they can't do. A fresh-from-Gauntlet magus may seem fairly potent by comparison to other RPGs, but they really aren't potent by Ars Magica standards. They are unlikely to have decent penetration totals, or if they do they will likely be one-trick ponies and will -need- support from others.
Some ideas for sagas focused on the magi:
- A group of junior wizards given responsibility for one wing of a much larger autumn covenant - being given regular missions and tasks by senior magi.
- A group of junior wizards in a semi-isolated situation but with plenty to explore on their doorstep, such as a base-camp outside an ancient ruined covenant that is extremely isolated, so help is hard to come by.
- Hermetic politics has already been mentioned
- A team of junior hoplites assigned to travel with a senior hoplite. Tasked with entering places and killing things, primarily.
- As above, only with a senior quaesitor instead - more investigations.
One thing I find lacking from easy reference for starting a game is a good reference to how potent a magus might get and at which stages of their lives. From my own observations, you can probably expect:
A magus fresh from gauntlet will have arts of around 7 in their five best, and nothing in the rest. A specialist might have their two primary arts up to 10, maybe even 12 at a push.
A magus at the end of their spring (~30 years post-apprenticeship) will have arts of around 14 for their five best arts, and 7 for all their other arts. Or if more specialist could have 20 in 2, 15 in 2 and the rest at 5.
A magus at the end of their summer (~60 years post-apprenticeship) will have arts of around 20 for their five best arts, and 10 for their other arts. Specialists might have their primary arts at around 25-28, with the rest still below 7.
A magus at the end of their autumn (~100 years post-apprenticeship) will have arts of around 24 in their five best arts, and 15ish in their other arts. Specialists will be skyrocketing up into the 30s or possibly even 40s with their primary arts, and even their weak arts are likely over 10.
These estimates are conservative. Chances are player magi will push past these figures - depending on the saga possibly well past these figures. The point of this is to highlight that fresh-from-gauntlet magi are scrubs - they will lose certamen more often than not, they will need to run and hide from someone playing wizards war against them - unless the person in question is of similar age.
It also means older magi are not going to treat them with respect. They will likely get about as much respect as a modern teenager gets from a senior law partner in most cases - i.e. roughly none.
As such, don't be afraid to treat them like level 1 characters. Have senior people send them out on adventures. Don't overload them with 2-3 adventures a season - let them have the time to study up and develop into older magi.
Edit: oh, and...
Have each player create (or pick a template) shield grog for their own magus and have that grog travel with them in most cases. Magi are squishy, don't be afraid to seriously wound or kill magi who don't take proper precautions. You don't need to have a fully detailed character sheet for each grog - my saga had the rule of if you didn't have a grog character sheet, your grog used one of the template stats from the core book. The up-side here was no character maintenance, the down-side no character progression.