I organized my accounts to have :
act 1 - tank chars
act 2- healers
act 3-5 - DPS

so with that I would do:
act 1 war, paladin, druid, rogue (not a tanking char, but should be leader if you want to box him)
act 2 druid, shaman, priest, paladin
act 3 mage, hunter, shaman, druid
act 4 mage, warlock, shaman, druid (or a spriest)
act 5 mage, warlock, shaman, druid (or a spriest)

The hardest group comp will be anything with the rogue. The way I would do that is to do rogue, 4xshaman or druid, and make the rogue the leader.

If speed is your priority, try to match up common classes in the groups. The group comps you have right now are pretty difficult to level.