im pretty sure that most hpi wheels/tires work once you have the 23mm hex conversion kit.
the only possible issue I could see, is if the hex of the hpi wheel sat too far into the rim, causing the rim to hit the XT's hub.
just measure the offset of the stock wheels.
as far as diameter is concerned, as long as it is with in 1/4 inch it wont matter any. greater than 1/2 inch either way you will notice a slight change in power or top speed (depending if it is taller or shorter)
edit, the guys in the giant scale forum could probly help you better. plenty of XT and MT owners there.