Congrats on the new puppies!!!
I can't help at all with the knee problem. I can tell you that the humping won't necessarily stop after neutering. It is more likely a dominance thing. My little Duke started that with our older dog almost right after we brought him home, and hasn't stopped since being neutered.
We actually find it hysterical at home cause our older dog is a lab/chow mix at 65lb. Duke rides her tail like a carnival ride
, his tongue is hanging out and everything. We discourage it, but it's still funny.
They will romp and play together that is normal.
Good Luck!!