[Cerek]

So you are taking two Rangers (Minsc and Kivan) and a Fighter/Cleric PC.

The first obvious need is for arcane magic spells. But you will also need a thief to detect traps and pick locks and you'll need a second cleric to help with healing. On that basis, your choices are....

Aerie - provides good assortment of mage spells and also adds extra clerical spells to your own.
Anomen - provides extra fighting skills and back-up clerical skills. Also very good at Turning Undead.
Jan - Mage spells and the only NPC that actually advances as a thief.
Nalia - advances more quickly as a mage than Aerie or Jan, but doesn't offer any help with thieving skills.
Kelsey - Mod Sorcerer. Pure mage spells.

Given the makeup of your party and your choice of Kivan and Minsc, I would suggest adding Anomen and Aerie. That will give you FOUR fighters to protect Aerie until she becomes the powerhouse she is capable of and provides 3 clerics for the group. One of you can handle Detecting Traps and Minsc can just bash open any locks you encounter.

You might add Jan to into the mix if you want the mage spells and advancing thieving skills. In that case, I again recommend Anomen to augment the fighting power and clerical needs.

So my recommendation for your group is...
PC (Fighter/Cleric)
Minsc
Kivan
Anomen
Aerie or Jan

Nalia is another option. With the first four above, you have enough fighter power to protect any other NPC. She will gain decent power as a mage and can either stay or go once you rescue Imoen (depending on whether you want to keep her or not).