Well if you were soloing them, the builds for the Paladin, Priest and Shaman would be Retribution, Shadow and Enhancement respectively. In a group, I'd be tempted not to have two melee characters as I suspect this would be harder to 'box but I have little experience so I'm just guessing. I'd suggest Retribution, Shadow and Elemental. You'll still have enough healing in your group even if healing is always the off-spec.

It's a long time since I played mages and the talent trees have undergone significant change since I did. I'd be tempted to level the mages on different specs though, mainly for shits and giggles.