You are exactly correct, the humping, tug of war and play biting is all about who's top dog in a friendly type of way. I have 5 dogs and my smallest, most demure female humps the older, much larger female dog. Both are best friends, it's just a playful sibling rivalry/pack type of thing. I usually see the younger dog doing these things to the older dog, or the dog which was in the home first.
Even the play biting, is, like with a kid, the dog seeing HOW far he can go to establish her higher pack placement.
You DO need to make sure they don't hurt each other in playfighting so it's good that you're concerned about that. It might not happen, but it's best to make sure and referee, because like little kids, they don't always know their own strength.
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