Ah, way to read the OP, go me.

Well with 2 you're going to have to rely on 3 others to fill a group. I'd still recommend going mixed class to facilitate this. It's also nice when a piece of loot drops to not have to decide who to give it to most of the time. Plus the whole synergy argument still applies. I think Shaman + Druid for example is > Shaman + Shaman. Prolly need to read up on what stacks now in 3.0, I'm not 100% on that stuff myself.