You'll struggle to get many keepers with your current setup.
I shoot hockey with an XSI w/ Sigma 70-200 2.8. I shoot in manual mode at 800ISO, f2.8, and can generally get 1/250 to 1/500 and find about 50% keeper rate. This is 6-10 year olds. For older, faster skaters, the rate would be much lower as you can't freeze the action at 1/250.
Are you trying to get action shots? Or individual shots of faces and emotion? If the former, use the 55-250 and expect a lot of PP to correct lighting issues. The players / coaches may not like you using flash and good luck shooting through the glass with reflections from flash. If you are going for the latter, try to shoot from penalty box, and get the kids as they skate towards you - use the 50mm and crop appropriately. Or shoot from behind the net, but you won't reach much past the front of the net.
I find 1600ISO on the XSi to be weak at best, so stick to 800. But, I'm blowing up to larger format (11 x 17 to 24 x 36). Your opinion may differ.
If you should in Av mode, set to overexpose by 1/3 at least. Shoot AI Servo.
Good luck
Brian