He should start at level 2, or even 1 if you go there very early.
IIRC, the level of party members should be chosen according to what level your PC is when you first enter the area they're in. I'm not sure what could have screwed this up, the fixpack definitely shouldn't have done so.
If you want to remove some of his xp to set him back to level 2, you could use Shadowkeeper, it's the easiest way.
One more thing: what level, or rather how much experience, should I except to be at by the time I'm done BG1?
If you have TotSC, then the cap was at 165k xp for that expansion, meaning level 8 for most classes (the cap for BG without the expansion was 95k xp).
However, since you're playing BGT, the cap is automatically that of ToB, which is 8 million xp. Given that there's a limited amount of xp in the game, however, you should end up with somewhere between 200 and 250 k xp if you do everything with a 6-member party.
And yes, as Shadowsetzer said, you will have to manually change Minsc's stats back to those of a level 2 ranger (20 hp, 19 base THAC0, remove 1 weapon point, etc...). It doesn't take that much longer, and you don't have to bother with any of the more complex stats like stealth or saves, since they don't matter all that much.