I did this a while ago.
If you are not using heirlooms you should make sure your characters don't end up ''stealing'' each others gear.
For example, if you go 5 druids, who only wear leather, your druid will get undergeared because of a shortage of leather gear.
What i did to counter this was having my warrior tank being the only plate wearer in the group and my healer the only one who used leather. This way you have the least possible chance of your most important toons getting undergeared (which they probably will be at some points because of 3x exp).
I think my team comp was something like this:
Prot warr (plate)
Resto shammy (mail)
Comb rogue (leather)
Frost mage (cloth)
Shadow priest (cloth)