I don't think Hojo would have been successful if Cloud and gang hadn't appeared. As Red XIII says, he was playing dumb so Hojo wouldn't be suspicious of him and he never had any intention of frolicking under Aeris' dress.

Then, even if they did happen to mate for some reason, Aeris wouldn't have been impregnated or the fetus would have been rejected extremely early on. They're genetically too far apart for it to work.

I just watched a TV show on whether Stalin was trying to breed a super-warrior race of monkey men and they went into a lot of detail about how, even though chimpanzees are genetically the closest monkey to humans, they still wouldn't have much success. Even if they were able to produce offspring, they would either be infertile/sterile or would die extremely young due to the different number of chromosomes.