You won't EVER want to stop the actors/actresses from rolling while you are shooting. If you try, you will not only interrupt their dress rehearsals, which are VERY important for them, but you will get crap shots. Sure, they might be positioned the way you want them, but you won't get the emotion and feeling that is the very thing that makes a good theatre shot.
This pretty much goes for all low light photography IMO, but here are a few tips:
1. Shoot highest ISO available. On the 350D, that means ISO1600. My 350D is my second body, and i use it 1600ISO all the time, and it is COMPLETELY useable.
2. Sacrifice DOF for shutter speed. A f16 shot is no good at 1/4 second shutter speed, but a f2.8 shot at 1/125 is far more likely to be usable. In my experience, solo shots are far better than group shots. Group shots are necessary alot, but i always love solo shots and rarely group shots.
3. Don't cut off fingers, toes, important bits. You will regret it later. You can always crop, but you can't expand on what you don't have.
There are more rules probably, but it is late 
Shooting theatre, concerts, and other low light events is not easy. If it was, everyone would do it, and we wouldn't spend so much money buying the best gear money can buy.
You are limited with your gear. Your nifty fifty is about all you can use, but learn to use it well, and you will get some awesome shots.
Practice makes perfect, as well as advice from POTN.
Rob