In my personal opinion the paladin would best be Holy, but otherwise thats an amazing set up really.
Melee classes have a tough time with Multi-Boxing because they face the target when attacking, which would cause them to stop following.
The bad thing about that in a set up like that would be that you have all casters, and then a ret paladin, unless of course the ret paladin was your main toon.
holy paly:Concentration Aura & Wisdom
Frost Mage: Snares & slowing targets
Shadowpriest: Dots & once higher mana/health battery
Ele shammy: Totems & Chain Lightning
Bal druid: 5% Crit rating.
Only cons to this would be the fact that you have a load of different spells with different cast times.
But I'd like to see how it turns out ^_^