As I only have one cat I may be not the right person to give any advice, but I do believe that cats love having a certain routine in their life and your own cats seem to have arranged that for themselves pretty good, but as you can see with Sterling and Jim further additions or changes have taken their toll.
I know how much helping cats means to you, so I voted to keep the kittens in a seperate room, because I think this means less stress for your own cats and you can keep up the good work with fostering.
I don't see anything against keeping the kittens in one room only, because that's more than they have in most of the shelters, plus you can give them a more thorough care and much love and most of the time you have more then one kitten, so they have company...
...but the thing is that you have to decide wether you can deal with this option (keeping them seperated)...I agree with moosmom that the permanent cats and their health and happiness should always be considered to be most important...