So I've been following the main storyline so far, and since Khalid and Jaheira have a meeting with the mayor in Nashkel I hurried there, not doing any side quests along the way.

I find Minsc in town, and when I recruit him, he's level 6. My current party of 5 (Myself, Imoen, K+J and Garrick) are all either level 1 or 2.

Does he just start at a high level, or did some fixpack up him to 6? Only mods I have are the basic BGT and fixpacks.

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.

Excellent, thanks!

One more thing: what level, or rather how much experience, should I except to be at by the time I'm done BG1?

I'm pretty sure you'll have to update his HP, THAC0, and weapon points as well. (Stealth as well, but that really doesn't matter.)

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.