Also, I'd suggest not boosting your first team. The whole fun of multiboxing is actually playing the team and its easier to learn on low levels (where dungeons are simpler). Boosting is fine for bringing up lots of alts quickly using RAF, but is itself a really not fun part of multiboxing.

Prot paladin + 4 shammies is probably the easiest place to start. Everyone wants to be a special snowflake, but there is a reason this is the default boxing group (some people say take a druid tank, but I prefer my happy shield-of-thrown-silence-bring-them-all-to-me spell, in general I like paladin tanks the most though). The synergy is strong and multiple chain heals can get you out of a lot of situations where another group just dies.

I don't personally prefer boomkins to try and heal with, I'd do four boom and one resto if you want to do druids (or 3boom, 1 resto, one tank) in dungeons. my attempts to heal with boomkins generally resulted in, first 3 oom boomkins and then one dead tank. Disclaimer, that group was 4 years ago during BC, I've only done mixed groups with druids since.