As for race: Draenei make good paldins because of their racial bonuses, but you don't benefit from having two in a group (talking about the + to hit bonus). I have a human and draenei tanking each of my two groups.
They make good priests too, but humans are nice because of the spirit bonus.
Boomkin is a N/E, no choice there . . .
Same with shaman, they have to be Draeni for now. I play warlock in my second group instead of a shaman.
I would suggest a dwarf for a hunter to fill out your diverse group!
Gnomes are cute for warlocks, but I play a human. They have a bonus to INT and engineering!