For heroics ther are few tankign classes.


If 5man heroic was my gole id go for:

1X Paladin = salv
3x Mage = BADASS nuke and sheep X3! :P
1x Shaman = Totems! o jea and that healing part XD


but if you wana go normals and then straight to Kara id go:

1X Warlock = Pet tanking!
4x Mage = sheep and real nuke

Speccem Demonology/Fire for instance and Destor/fire for Raids (25% more firedmg is awsome!)