To me it's not entirely related. Does he get along with other dogs he sees regularly? It could be he's nervous, especially if he's not well socialized. This type of thing is usually more about the dynamics between adult dogs and puppies. Something to consider. Adults dogs often don't tolerate puppy antics. They don't like being jumped on, mouthed, and wrestled with all the time like puppies do. It's pretty typical for an adult dog to "put a puppy in its place" by baring teeth, growling, looking away, turning their back, or ignoring the puppy. That's how dogs teach puppies. Puppies need to learn those social cues mean "leave me alone, I don't want to play right now." You should ALWAYS supervise and make sure the puppy gets the hint. Fights are never ok. It's fairly normal to have little scuffles that last a second or two when introducing a dog of any age into a home that already has dogs but fullblown fights are not ok and it should never escalate to that point. Personally I let my dogs give warning cues. I have a foster boxer right now. Both my dogs will show teeth, grumble, do air-bites, or turn away from him when he wants to play and they don't or when he gets in their space and they want him to back off (they are 2 and 1/2 years old, he is 1 and 1/2). It's social behavior and I like that my dogs display their feelings. You should NEVER punish growls or teeth baring. If you try to correct him or punish him for doing that he will learn to skip those warnings and go straight for a bite.