Gonna need more info than this to help you out. Which XT RD are you talking about - 9 or 10 speed? Any Shimano 9 speed rear derailleur should work fine with any road shifters (as long as they're not 11 speed shifters). If the RD is 10 speed mountain, it won't work with any road shifters. What shifters are you using?
Ack - you beat me to the reply!!!!
We need a little more info to help you out. If you are using road shifters then a shimano XT RD will not work if it is 10 speed. If it is a 9-speed it should shift fine with Shimano Road shifters. That derailleur will handle any cassette you want. If you are running a single ring (36) the derailleur capacity is irrelevant. You do not need a Shimano mtb rear if your largest cog is 28 - all of their road derailleurs will handle a 28. Give us more info and the forum will get you set up.