If it is your first team, I'd go with one of these compositions.

5x Elemental Shaman
4x Elemental Shaman + 1x Paladin (Ret 1-39, Prot 40+)
5x Balance Druid.

If you think you'd like to pvp on the team, Shammies are more successful in Arena, Druids are just as viable for world/BG's.

Down the road, one of the shammies/druids could easily dual-spec or respec Resto, if you'd like to do heroics.
With the shammy team, a pally lets you do heroics.
With the druid team, one of them can dual/re-spec to feral and a bear is a strong tank too.

If you go with the mixed pally/shammy team, the pally is a weak link in pvp's.
You could run 4x shammies and sit the pally out.
You could spec the pally to heals and run with the group.
The more diversity you have the harder it will be to use it all, particularly in pvp's.
4x shammies, with someone else playing a healer toon has had a lot of success in arena.

I'm not saying don't go with a mixed team.
Just that a mixed team is quite a bit harder, especially for a first team.