Curbing puppy behavior

I feel you have a right to be concerned, considering the breed
characteristics that you believe is in the ancestry of this dog. As Matt
(Rocky the dog) said,I would not allow them together unsupervised -ever.
But, if it makes you feel better, puppies DO that.
I have a 7 month old puppy. He plays so hard with my 5 years old (who plays
equally as hard back!!) that it's just scary sometimes. neither are dog
aggressive, and obviously are having fun. But when I think it get's too
much, I put the brakes on their party.

This weekend, my sisters dogs were playing with her son's dogs. The smaller
of the two, ran full tilt into the bigger dog, knocking the wind out of
her. It was good natured play. My sister called last night and said her dog
had a fractured rib out of the ordeal. It was lucky it didn't puncture a
lung.

I am a new subscriber and also a new puppy "owner" and have a question.
I currently have two adult dogs (ages 2 1/2 and 4ish, pit bull mix and
lab/husky respectively). I found a puppy a few days ago and plan to
foster him until we can find him a home. He is roughly 13 weeks old
and 13 pounds. He seems to be well mannered etc and is picking up on a
lot of our training. However, he is starting to play very rough with
our younger dog, and she is getting rough with him. It is all growling
and no bite, and I am not worried about the puppy, Elmer, getting hurt.

Rocky...

The younger dog is the pit bull mix, right? What's her history
of playing well with others? How well can you read her
intentions?

I deal with a fair number of adult/puppy interactions; they
often sound harsh, but it's usually play. The key word being
"usually," I wouldn't leave them alone even for a moment.

What I am worried about is that I am tolerating aggressive/dominant
behavior on his part and that this may lead to more intolerable
behavior in the future. He is the instigator, and pushes our younger
adult dog until she responds. How much is "just puppy behavior", and
how do I know when the line has been crossed? I have looked in several
books, but don't see any advice on introducing a puppy into a currently
dog-occupied household.