I love good Thievery, when it's well-done. Radiant_Mephit shares my opinion of having 2 Thieves: One for Fightin' and one for Stealin'. That way, you get all your bases covered quickly, you can set twice as many Traps, and you enjoy full Backstabbing benfits, like taking out Bone Golems with one swing.

As for the Cat Familiar: My plan was to make a Chaotic Neutral Elven Mage/Thief, and have him concentrate on Find Traps, Set Traps, and Locks. Enter the Cat, which has 99% in Hide and Move Silently. So from the very beginning of BG2, this guy has 5 of the 7 Thieving skills already at 100%!

Party of Three ideas:
Berserker->Cleric or Undead Hunter
Fighter/Thief
Cleric/Mage or Swashbuckler->Mage