Wrestling is a dominance struggle - but full-out fighting is serious. If this was really a fight, and the puppy was bleeding, I'd be very concerned, especially them being so young. Watch them together some more and see their behaviour towards eachother. Mounting, playing, wrestling, etc, are all forms of working out a pecking order. But fighting to the point of blood is a form of aggression and I would take it seriously. If that's what is going on, I would take my puppy to group classes as soon as he is old enough. Do you have any friends with dogs that you know have had shots, that could come over and play with Bull or Shadow? Socialization is the key! Maybe all that needs to happen is for one of the pups to be put in it's place before the fighting stops. Good luck.