I've seen/ridden TB/draft crosses before. My last baby Shania was a TB/Draft and massive just like your new boy. I just think a paint..usually just over 14 hands, would have a tough time with a 19+ hand stallion.
Would it have to be a live cover? Or could you do AI?
Your new boy is VERY flashy, hard to tell from a pic but he looks like a nice mover(would love to see a video)Hope everything goes well with his final testing. Does your mare have colour? Does she throw colour? I think its a gamble either way you look at it, if you had a heavy mare and bred to a smaller stallion, you could still end up with a short legged thick horse/pony without colour.