Theres a reason everyone does shamans, its because they work out really good for both PvE and PvP - a lot better than any other 5 of a single class/spec I would imagine.

As far as druids, it depends on how you plan to use them. 5 moonkins? A feral, 3 moonkins and a tree? or what?

Seems like it would be a lot more hassle and a lot more concentration to 5 man an instance with 5 druids, compared to 5 shamans or 4 shamans + paladin but I'm sure it could be done.