I would gradually try to put Sterling and Jim off their medication and see how they react then, for my main concern would be to have them on medication on the long run...

I wouldn't let the kittens interact with them (Sterling and Jim) tough...you've posted that they have enough space in your home and they aren't alone in there...instead I would try to install a cat door to their foster room(s) with a four way system that allows other cats to come in, but the kittens cannot go out...so the kittens can interact with the cats who want to come in on their own choice...

Just my two cents...