There you go 
Mister Link beat The Fixer to it. Yet I shall still await his confirmation and judgment, for such is the authority I have invested in him.
Marko?
It's a mess.
It always add a characteristic, but it depends on the situation. Per to hit, Qik to fast-cast, and for the quality, it's either Intelligence or Perception. Callen and I had a small discussion about it once, on these boards IIRC, and it seems that there's some consistency to it, but I can't recal what, if any.
There:
That'd be Int for Creo, Per for finesse, mostly. Some of the time. If the stars are right.
As said on the other thread, 2 things are clear: Aiming uses Per, Fast-Casting is Qik.
Other than that? No hard and fast rule, and it's a mess.
I tend to agree with JL for Per/Rego and Int/Creo, but this is more a HR than anything else, although I believe Int + Creo was slightly clarified somewhere, probably in Societates.
As an exemple Callen found exemples of Craft magic using Int, I remember some using Dex, and I even think I saw one using Com 
At least, Int + Creo seems sturdier than Whatever + Rego . And there's the other forms 
Ummmm....
Which one is that again?
Heh, you have had rather an influx haven't you?
Bernat del Cavall is my companion character, a spy from the court of the Bishop of Urgell, fleeing for his life after he discovered that the Bishop is treating with the Fae.
Hm, that's a dev thread. I should post him as finished.
I was adding his Per, which is +2. My (apparently incorrect) assumption was that Per is used for all Finesse rolls, which is part of the reason Vocis's Per is higher than his Sta. It sounds like most of what he does is either Per or Int (also +2), so he's good either way.
Official generic AM coins are 10 to the pound, and gold trades silver at 10:1, so 1 gold coin is 1 pound of silver equals 10 silver coins.
which puts official weight of all coins at 1.6 ounces, if you want to try and factor in real world currency, though as noted previously, he gold exchange is off.
I don't know where any of this comes from. I am trying to find a source to site. ut in Ars Magica, it is 360 Mythic Pennies to the Mythic Pound. Gold trades to silver at roughly 16 to 1, more or less. Not sure on that, Ars Magica deals only in silver and not Gold.
What character or characters is he associated with?
None, a priori. He is a free agent seeking sanctuary with a group he has heard of in his professional capacity.
You might work out best picking up a job as a mercenary or a laborer. Wanna be a sailor? We can add you into Moonlight over Giza and say you had been a redshirt laborer working under Ximo and Jimo.
Hmm, I rather liked the idea that he deliberately got himself mixed up with this band of freaky magi; but I guess this could work too. He could be looking to flee the country, although with no magical transport he's a long way from home. Still, I could see that.
Would it be plausible for Bernat to have stowed away on Johan's boat?
I actually kind of like your original idea as well. I think Marko would be willing to run with it if you flesh it out a bit. Would he show up at the doors of the covenant, try to contact them via a messenger, or something else?
Selim would love to join this "buddy movie" 
Anything that entails travel and trading definitely fits within his interests. But, if you want to keep it a magi only thing, that works too.
Yeah, yeah. Flesh it out. I am just trying to shoehorn you in, so any better idea you guys have is preferable.
Ok, fleshing it is.
Is it possible to get into the covenant, as a mundane, without being let in? I rather like the idea that he arrives in the dead of night, with heavy rain, breaks in looking for shelter, and is found in the morning by someone - but if that Just Doesn't Work, it's fine.
Selim would be a great addition - a third cutthroat trader with a third perspective on the world. Lots of social interaction opportunities there. But I'm not the one running it. 
What sort of trade would they be looking to engage in?
They are obviously going to Jerez to get some sweet wine for the magi. Only problem is that there are quite a few not-so-friendly people artoud, with the Calatrava knights, the Portuguese, the local Muslim powers and the local djinn all vying for dominance and hating each other. You get both forest, grassland and desert in the area. And bandits, there must be bandits around as well. Wasn't Cadiz where all those ghosts from legends of Hermes were located?