By default, I think BG T limits you to the xp cap from the expansion of the second game, Throne of Bhaal, which is 8 million xp, about level 30 for most characters.
After all, BGT makes the whole series one big game, and there's no way for them to set different xp caps for different parts of the game.
OTOH, there just isn't that much xp available in BG 1 + TotSC, so you probably won't end up too high to lose all challenge in the game. If you really want to set appropriate xp caps, maybe you could use this mod
, it gives you several convenience options, including changing the xp cap.
Of course, you'd have to uninstall and reinstall it for the BG 2 part of the saga.