If you're going shammies, I'd have them all Elemental.
It's good damage, and they're good enough healers for most situations.

If you were going with a tank and a dedicated healer, I'd really recommend a Dreamstate Druid.
Mainly because you're capable of strong damage and strong heals in one-spec.
Gearing is pretty simple, you scale off of INT, SPT and Spell Power.
Druid + Tank + Ranged DPS would be a pretty good three-some.

All the same class, Shammies all the way.
Boomkins are a decent option too, but probably not as strong.
I like that with dual-specialization in the expansion... you can have primary and secondary spec.
So you could go Boomkin-Feral, Boomkin-Feral, Dreamstate-Feral.
Stealth to whatever you want and blow it up. Or alternatively have a half-decent tank set on one of more toons and play it safe with Tank, Dps and Heals.