The 5 druid idea sounds solid. As you mentioned, DPS will be an issue, but if you are going to quest level then it doesn't matter. If you are going to instance level, then you just need to wait a level or 2 above the recommended.

As for a druid tank, definetly possible to tank anything in the game. It all comes down to gear. If a shaman can tank Kara, then a druid can tank heroics.

When leveling up to 70, spec 4 of the druids balance. You will not need a dedicated healer that early on. This should be able to provide more than enough healing using a round robin rotation with the 4 druids and increase the DPS until you get boomkin



On a side note, does the boomkin aura stack?