Hello,
This is normal behavior for dogs. The humping is a sign of dominance and it seems that mini is trying to show the bigger guy that he is boss. =) If the dogs are not neutered it would be wise to do so. This should curb the behaviors somewhat. As long as they are not fighting then there is no need to keep them apart but I would keep a close watch on them just to make sure they are getting along. I have raised pits myself and although mine were wonderful, sweet and very tolerant, they are big, strong pups and if a fight should break out the smaller guy could get hurt.
Hope all goes well!
LadySaja is right! Licking is a form of submission towards the one being licked and the humping could have started as dominance and now it could just be something to do, or still be dominance. Unless they are fighting it's nothing to worry about!