We've used 9mm HTD belts for the last 8 years no problem. GT2 might give you a few problems. Small pulleys will do the same. I believe we run 36t pulleys for all of our wheels. We've used them for our standard 6" wheels, and 8" wheels as well. HTD is better for the quick change in direction. HTD is High Torque Drive.
Try using SDP-SI's belt distance calculator. It'll give you pretty good numbers to run with. We also pull all of our pulley files from SDP-SI for our CAD.
http://www.sdp-si.com/cd/default.htm