Music for more than a quartet, less than an orchestra?

Recently I've started playing with a few others, and while we're having fun playing together, we're having trouble finding music to play for our setup. We are 4 violins of rather diverse skill levels, two violas and are possibly soon joined by a cello.

For now we've been playing material out of Carl Fischer's Progressive Quartets, but there's always the problem of having more instruments than available voices. I suppose we could simply have two violins play the same voice, but I'm thinking that would create a bit of an imbalance between the voices.

I suppose we could try either finding music with more voices, or have multiple instruments on each voice... what would be your strategies of dealing with that?