Humping is your dog's way of trying to establish dominance with your neighbor's dog. If the neighbor's dog submits to him, then he will soon stop humping her. If not, it may continue, he may let it go and submit to her, or it may escalate.
Your dog growling was his way of telling the other dog to back off without it escalating. As long as she listens, this isn't a problem.
The biting that your neighbor's dog is doing is her way of getting attention from either your dog or a dog at the park. She is being playful, but it may be annoying to some other dogs. An older dog will correct a younger dog appropriately (slight growl), but an unsocialized dog may be too harsh (snapping) and a fight could break out.
As long as the younger dog is learning how to behave around the older ones, this behavior isn't so concerning. Supervise playtime and if it looks like your dog is getting tired of the little one or has "had enough," end it and play again another day. Keep playtime a positive experience and you shouldn't have any problems. Always supervise, because something could go wrong in less than a second.
Courage is the magic that turns dreams into reality.