[Maximo]

Definitely keep the pup with the open font separated from the other dogs at all times.

My boys play wrestle often and for the first year they were together, it would occasionally get out of hand because the older one would get fed up with the younger one who didn't know when to quit. It always looked and sounded scarier than what it really was, but I have the advantage of my boys being similar in size. They also run right over each when playing fetch.

With one dog 3 times the size of the other, you do have to exercise caution, supervise and call time outs. I would separate them when you cannot supervise.

Are they neutered? If not, that will escalate your problems.

Whether or not things improve with age depends on the individual dogs. My younger guy has mellowed and has learned when to back off.