OP, do you feel that they are playing too rough? Proper play should be mutual, with both dogs taking turns at chasing/being chased, etc. If the play makes you uncomfortable, break it up, redirect the dogs to something else and try again later.
"Popping" the pup isn't going to serve any useful purpose and will cause mistrust between you and the pup. "Punishment" only serves to cause confusion unless the pup can make an association between what you perceived as "wrong" and the "correction". So I'd avoid punishing a dog for something he doesn't understand is wrong.
I don't do a two week shut down for foster dogs that age but I DO carefully supervise and monitor any interaction and I give plenty of time apart. We use a crate and rotate system until everyone is 100% comfortable with one another. This can take weeks or months of intervening, interrupting, rewarding and separating. I have two resident dogs. We rotate and take turns and when we need to, either the pup or the resident dogs get a time out in a bedroom with a chewie just to relax and decompress.
Given the growling that your dog is doing when the foster has a treat in his crate beside her, I don't think they have adjusted to one another as well as you think they have. I would take that behavior as warning and back up a little. My theory is that it's always better to be over-cautious and avoid a scuffle than to throw them together and wonder why they ended up fighting.
Thanks for fostering.