It's a lovely piece, a little on the short side I think. I agree with Sojar about you experimenting with more adventurous harmonies...just experiment with it, I don't think there is a fool-proof method.
Practical performance question: are the soloists suppose to be section leaders, or seated separately? I think the soloists should play the tutti parts when not playing the solo stuff (it seems more practical that way, especially for the viola and cello).