Would you be getting the mastiff as a puppy? That could make some difference because he may learn that your other dog is alpha while small and be ok with it, also he might me able to ajust to it more as a pup. As an adult that is already and established alpha then thats when you would have possible problems. Also from what I notice, some big dogs don't mind being bossed by a smaller dog, I knew one that feared the smaller dominate dog. It could just come down to the dogs individual personalities. I don't know a whole lot about this, but I have a problem with a dominate female and this is the jist of what the vet told me when considering getting another dog. Hope I was somewhat of a help.