I would take them to a pet trainer before that mastiff gets bigger and older. He will grow to almost full growth by 6 months. For now, never leave them alone together. The puppy is young right now but once he is full grown you will have a difficult time controlling it if you don’t learn to handle him now. Till you can get a trainer. (try petsmart) Try holding him down on her down on her back whenever you see unwanted behavior. This is a submissive pose and she will at least get the idea that she is not the pack leader and you are. Hold her down till she is calm and hold her by the neck. This is behavior that other dogs do with each other to teach who is leader and boss of the pack. You may want to do the same with the pom. Only make sure to do it when you see them doing something you dislike. Like aggressive behavior. They will get the idea in time it is something you do not desire or will tolerate. You must be consistant and don’t let any aggressive behavior pass because it is cute. With each other, this cute behavior can lead to their death. Mastiff are usually very calm dogs but poms have a Napleaon complex. You may want to work a lot with the pom. If he is not fixed, you may want to get him fixed. This will calm him down a little. But do get a proper instructor. They probably have several tips on handling the problem and can show you what you may be doing wrong that may make the problem worse than it need be. Good luck. Hope it works out.