The 16T vs 20T determines the diameter of the pulleys. The bigger ones may not fit. This number also applies to smooth pulleys without teeth (which is counter intuitive). The wrong size here probably won’t fit.
The 6mm GT2 belt (GT2 means 2mm between teeth on the belt) will ride on 10mm pulleys, with 4mm of extra room. It will work fine. 9 or 10mm belt is just stronger and it stretches less. We used 6mm belts for years until 10mm belts were common.