First off, the pitch of the belt/pulley has no relation to the pitch of the ballscrew. So that is not a concern. If you look around on the SDP-SI site, you will find various numbers for the allowable working tension for the sizes of belts. As you have not given us any feel for the size of your machine, it is hard to give any real numbers for pitch and belt width.
Now having said that, on my machine (RF-31 mill/drill), I used XL 037 belts and pulleys. I did this based on the belief that it was strong enough to do the job ( and so far it has, I have snapped off 5/8" endmills as G0 speeds when hitting the vice) but it can also act as a "fuse" or breakable element in case things go pear shaped! This has also been proven one time when I had a hard crash of the Y axis and the $4.00 belt broke before breaking or bending the ball screw or worse!.
As for tooth form, I have had no backlash problems with the trapezoidal tooth form, but others will swear by the HTD or GT2, The Tx series are similar to the trapezoidal form, but metric measurements. When sizing the pulleys, one thing to remember is you want probably a minimum of 6 tooth belt engagement to minimize the probability of slipping.
With a little attention to the setup, you will find the belting will be as accurate as the direct drive you now get if not more depending on the encoders you use.