My two will play fight - it is their way of burning up inside energy...and outside too. But I think what the others said is true. This was more a matter of them trying to figure out who was the 'dominant' dog and who was going to be submissive. The mounting is not to worry, I have known female dogs that have mounted like a male, and shocked people just because they wanted the upper hand ; ) no pun intended.
Just remember to keep an eye on what is happening.