[Faceman]

As you obviously have some very good people in your party (alignment-wise) it would be harsh to reject Imoen. However until then or if you choose to ditch her.
If you want a tank:
1. Mazzy: Can be used as tank and as support (with the Machinegun-Tuigan) and has nice interactions with the other Paladins.
2. Valygar: Backstab! NIce equipment for start. Gets on my nerves from time to time
3. Minsc: Tough and cool but I have the feeling somehow that he doesn't fit into your party as you have nobody he has a lot of banter with.

For a mage:

1. Jan: Brings fun into the party and is a more powerful mage than most want to admit. I concur that he is only multi-class but inspite of this drawback he rocks. Also he is a thief and you are missing one right now.
2. Aerie: For the full spell selection. So you'd have 2.5 mages and 1.5 clerics.
3. Nalia: With a lot of potions she can do the thief tasks and you'd have another straight mage.