For a 10-man group there are several considerations.

As you add a more diverse mix of characters, your efficiency per toon goes down.
At the same time, you gain additional buffs/procs and tools to tackle more challenges.

You could easily run...
2x Prot Pally
8x Shaman, some dual-spec'd to Resto too.

It would be a very simple group as far as macro's and such go.
The team would run much closer to maximum efficiency then one of every class.

Personally, if I was going with a mixed (10-box) team, I'd look at:
- Covering as many buffs/procs as possible.
- Desired loot, so there's little overlap/competition, and not much is wasted.
- Which classes can be macro'd for close to full strength on the fewest keys.