As long as your Jack Russel does not have small dog syndrome, there are plenty of "Molossers" that would no doubt love to play with him... Not all of them are tough guys. Far from it. English Mastiffs, Bullmastiffs, St Bernards, Great Danes, Irish Wolfhounds, and Newfoundlands are all Molossers and they are mostly gentle giants. In fact, I have seen the Newfoundland referred to as the worlds most gentle breed of dog.