Mountjoy isn't a medium because if his powers. His only 'non-mounting' power is just to dampen reflexes at a short range. That's just a a reg power. Mountjoy's a middleweight because he can pretty much copy anyone's powers, as well as remove them from the battlefield. And by copying powers, even of just a couple of regs, he can become a medium himself. (Mountjoy would be pretty useless as a regular because he wouldn't be able to go up to middleweight without his owner sacrificing their middleweight, or not putting one in their lineup.)