Got ya. Well obviously if you selectively quest to avoid collections your group compositions don't really matter. Just go all DPS.

On the other hand, if you want to level through instances, I'm going to make some assumptions, i.e, you want a holy trinity with no melee dps. With that said, you ignore the rogue leaving you with 35 toons/7 teams. Also that means your DKs and Warrior have to be tanks since they can't heal or do ranged DPS which gives you 6 tanks. For your 7th tank, I'd pick one of the pallys. Now your pallys can't do ranged DPS either so the remaining 4 would be healers leaving you with the option of priest/druid/shaman for your other 3 healers and then everythnig else would be ranged DPS. Mix and match at your leisure.

However, as seen on these forums, people are running tank/4 dps, healer/4 dps and melee dps setups so your options are pretty much endless.