Also in my experience, the shallow DoF with a nifty-fifty opened right up doesn't help with useful clarity when you're trying to capture a moving target. For sure something will be perfectly in focus, but it may not be the bit you want.
I'd typically use my good old 18-55, running at ISO 1600 (which is remarkably good on my 500D) at -2/3 to -1ev. With good stage lighting on the important subjects then should work out as long as you can stay still enough!