Im in the same boat with the dancing photos. My daughter is on a travel team and I've been fortunate (or unfortunate) enough to have shot in a lot of different stage lightings for dance. The main lens I use is a Sigma 70-200 2.8 non is. I have a 85 1.8 and a 50 1.4 but have found that those aren't that great depending on where you are. The zoom gives you more versatility if you are locked into one place. With your 7D, you have more than enough camera to handle the poor lighting you may run into.
These were shot with a 1d3 and the 70-200