The only way I can manage the different diets is to build some cages (like rabbit cages -- just simple wood and wire with a hinged door and a snap lock) put food out three or four times a day (since cats like to nibble frequently) and then take it up. Meanwhile, during the mass feeding moments, I lock up my special eaters with their own food. Kitten food appears to be extremely enticing to cats of all ages or ailements. I have at present 17 cats, and most of them are on regular food, but I have 4 kittens, one elderly cat, and one cat who requires a special diet for preventing crystals in his urine. I also have a diabetic cat (insipidus), but that doesn't require any special food -- just the eyedrops every day.
I hope others will chime in on how they manage this problem. If you only have one or two cats, the situation is usually pretty easy (feed the kittens in the bathroom, and don't worry if they nibble on other cats' food, since it probably won't hurt them.
You know, after you insist on feeding the cats at set times and in set places (and you establish a loud call to gather them for feeding), you will find that the cats learn fairly quickly where they are supposed to eat. But if there is no fencing between them, they will automatically grab each other's food just because they are both curious and competitive. If you keep them well fed, you won't have them eating too much in the neighbor's yard -- I have a neighbor who throws out all their dinner scraps into the garden, and my cats naturally feel compelled to snatch some of it from the wild cats. I am shortly going to solve that with a proper cat-escape-proof fence.
Whatever solution you discover, please share it. I could use some clever ideas, too!