Depends on the build and age. My friend has a Appaloosa X ID but he is only 14.2hh and is 4 yrs old. He has 8" of bone so we wouldn't put more that 10st on him. Maybe when he is older he could carry more.
A good solid old fashioned D can carry 14 stone easily. 15st friend used to ride his D stallion regularly with no problem. The more modern riding types perhaps somewhat less for arduous exercise - but they are tough beasties.
Dunno about a spotty cross - depend on bone/substance/conformation.