The most common choice for mostly one toon type seems to be:

5x Shaman
4x Shaman and 1x Paladin (or other Tank).
5x Paladin
5x Druid
5x Death Knight

If you have aspirations towards PvP, the fewer classes present the closer to optimal you can play them.

During RAF, I leveled Druids (Balance), Hunters, Priests (Disc) and Shamans.
Already had a dozen or so toons at 70th, to fill in here or there.

A mixed team brings a lot more to the table, but the macro's and optimal control are harder to attain.
Look towards the castable buffs and proc buffs they bring to the team.
Look towards the type of loot they'd like.

A Disc Priest, Warlock, Mage, Rogue, Feral Druid could work, but 3 of them are fighting over the same cloth gear, and 2 on leather.
A Paladin (prot), Shaman (resto), Druid (balance), Priest (shadow) and say Hunter.. will waste a lot less gear.

If you're doing a mixed group, your focus is probably PvE - Heroics and maybe boxing raids.
That doesn't mean you cannot do BG's, World PvP or some arena for points.

Anything you'd like to try will work to a degree.
With the "Click to Move" and "Interact With Target", melee groups actually work fairly well and much more easily then previous.