That's a tough question. The shaman chain heal from three can do a lot of healing even without having a healy spec. I leveled a four hunter/druid team to 70, but when I got to 70, I swapped out the druid for my healy Shaman. I really like chain heal as it's easy to just hit one button and get multiple targets as opposed to targeting around to put HoTs from a Druid. My current team is a pally/4 Shaman setup and the shaman are all elemental, but they do healing just fine. Hey, 4X regular heals will do the trick.

So to answer your question, I'd go with a Boomkin and three Shaman. I believe you're going to have a tough time as it is with three different classes. I don't see how you're going to control your group with anything other than the warrior and you're going to spend a lot of time trying to hold agro off those other guys throwing down big spell damage. That's why I love the pally as a tank. I basically consecrate and that's all that's required.

Good Luck,

Johnny