I got my first dancebelt many years ago when I was about 13, and remember the experience. Back then it was a rubberized elastic. Some might consider it "primitive" today.
As for the thong strap, you don't need to pull it up so tight. After you put the dancebelt on tug a little on the thong strap so there's some room and it does not fit too tight.
The other problem is kind of delicate and depends on your anatomy. The first response would be something like pull the front up tighter, but that will also pull up the thong strap. You may consider placing a little pad of material under your testicles to fill out the pouch more. Maybe other posters here will give you other solutions.
Good luck and keep dancing.
balletboy[/i]