In BG1, I think it depends a lot on the alignment of the protagonist I am playing. If I am good, I go with the canonical party. If I am evil, I generally go with (now, with 99% less spoilers!) evil cleric, evil fighter 1, evil fighter 2, evil mage and Imoen. Regardless of who I play, I ALWAYS keep Imoen with me... I just can't get rid of her.
As far as the characters always joining you at your level, I just instll the level1npc mod. They always start with the roughly same experience that I have, but they begin unleveled up (or at least as unleveled as the game allows for). So, imoen will be level 1, but some of the characters you meet in Chapter 4 or 5 might start out as level 4 or 5. At least this way I get a at least some input in how they are developed.
Also, there is another mod (included in the tweakpack for TuTu, iirc) that allows you to use the Freindly Arm Inn as a sort of Copper Coronet for the first game. Most of the good and neutral character can be sent there, and the evil ones mostly go to the Nashkel bar.
For BG2, I shuffle my characters around a lot. If I have a romance going, I keep that character in the party, but the rest are mixed and matched depending a lot on alignment.