No one ever said this was a PVP focused game, and it is not. It is a highly PVE focused game (re: The Foundry). However, in PVP or PVE, multiclassing is a balancing nightmare, and frankly, better left out.
Keep it 10, maybe 12 man if you want to stretch it. I believe these are the ideal numbers for challenging content where everyone has to pull their weight. Five man content can be just as epic, but I agree a few more friends make the scale seem a bit more severe. Personally, my favorite approach has been Rift's 10 man raids…