[Nerull]

Honestly, you don't need 2 thieves, so choose between Valen and Jan (if planning to roleplay really evil, go Valen; she forces you to play evil). And to be honest, if you give Korgan regenerating items, you don't need another tank (you can always have Viconia back him up, especially once you get a strength belt for her). YOU HAVE 3 MAGES! Use them! Boost Korgan to ridiculous levels before combat; with 2 clerics and 3 mages in the party it's not like that is a problem.

If you are insistent on adding a 6th member, then Dundee is correct on Anomen. He can wear heavy armor, put 5 stars in weapons, and buff himself with clerical spells (you just have to make him fail his test). Jaheira is good, too, and Haer'Dalis is good if you use his spells to buff right (four wizard spell casters in the same party?).

I am not impressed with the selection of neutral and evil fighters in the game. If I could program, I would create a Chaotic Neutral Half-Orc Barbarian (use that one stray half-orc picture that comes with the game).