When it comes to play fighting, the key is deliberate intent. If two dogs are wrestling and one dog accidentally goes a little overboard, you should see that dog immediately back off when the other dog gives a little yelp, for example. That's if the offending dog has no deliberate intent to hurt. If the offending dog doesn't back off, there's cause for concern. So, my encouragement is to first establish the boundaries of acceptable and unacceptable behavior and then simply monitor your dogs' adherence to these rules, stepping in only when necessary. You as the head of the hierarchical structure need to establish clear rules, especially the rule that aggressive behavior is absolutely unacceptable. Dogs with a strong human authority soon realize that it's best to live in harmony and be subservient rather than face the consequences of an unhappy leader. Dogs do this by testing one another's dominance and subservience, mostly through play and general interaction. They wrestle, tug, mouth, pounce, play bite, and even softly growl and yelp. And, this is all okay - as long as the dogs maintain self-control and do not allow their behavior to escalate into outright aggression.