Very hard decision.

Here's my personal feeling and two cents, take it for what it's worth. If there was no health issure causing the spraying I think Jim and Sterling were trying to tell you there are too many cats for their liking and I'd hate to see the fosters cause them stress (when it isn't absolutely neccessary) and they have to be confined again.

I think if you are truly torn between this you should decided who is more important, Jim and Sterling or the foster kittens. It sounds to me like your fosters have adequete space, especially considering the alternative of a cage. Since Jim and Sterling are your cats you owe it to them to do what is in their best interest. On that note, I don't think the kitty Prozac is the answer to everything and personally I would try to wean them off of it and keep the kittens away. If they start spraying again without the meds then you know something besides the kittens is bothering them.

As for the kittens, could you let some of your other cats in to see them off and on?