I think a tank is all you need. The shammies obviously don't have any damage mitigation to speak of so go down fast if one of them becomes the 'tank' A pali tank is easy to integrate into your team. You can either micromanage it if you like, or just spam a castrandom macro. (I prefer the former most of the time).

I have not gone with a dedicated healer. My reasoning is that 4 lesser healing waves + healing stream totems seem to be sufficient to keep the pali topped off. He can always bubble in an "oh-shit" situation, and because the damage he takes decreases as his health gets lower, it's really a no brainer. Since the healing is spread among 4 toons, none of them has to worry about aggro from healing and I don't have to worry about my dedicated healer getting CC'd. I can throw a 4x chain heal in there to deal with AoE if needed. I doubt it would work in arena, but for PvE instances, it's been fine. I have given some thought to subbing a boomkin in for one ele shammie for the buff and the instant HoT, as others have done.