As a few have said both female and male dogs can and do mount other dogs to show dominance. I had a spayed female and neutered male for 12 years and she used to mount him when she wanted to tell him she didn't like what he was doing. It was hilarious to watch because she behaved like the bitch she was toward him each time he paid attention to another dog.
She was the queen of the house, we had 4 cats as well, and she kept them all in line by showing them that she was the dominant member of the pack. With the cats she'd push them apart with her nose whenever they were play fighting and her treatment of our male dog reminded me much of husband and wife. They were the best of friends too but she was definitely 'alpha' in the pack
The fact that your puppy is showing her belly to him shows that she accepts him as the dominant member of the pack IMHO.
