The situation now is that we have a 6 year old cat (no breed) with his special dietary food and a small kitten of 5 months (Mainecoon / Norwegian forest cat mix) with her special kitten food. We don't have a feeding schedule as the cats are not food obsessed and eat their bowl throughout the day. As the kitten food is much tastier than diet food, our big cat tries to get to it, but for now we have made a box for the kitten's food where only a small cat can get in. But as she will get much bigger, the box system won't work. Also the kitten sometimes eats the big cat's food seemingly just for company. So the question is what to do in that situation? Do we make a feeding schedule for the small cat or both of them? And how to go through that slowly or just start different feeding?
I recently had to start feeding both of my two cats different food. I accomplished this by switching from leaving food out all day to feeding them twice a day. It was much easier than I expected. When I put the food out, I just make sure each cat starts with their own bowl; I find they stick to their bowl and don't try to switch. I don't have to stand over them and enforce it.
I thought my cats would start waking me up for the morning feed, but they don't. I think that's because I don't serve them breakfast until about a half-hour after I wake up. Also, I give them more food in the evenings, so they aren't ravenous in the morning.
There are mechanised bowls that will open only for a specific cat. Some work by detecting the cat's microchip, so your cats would need to be microchipped. Others work by detecting a special tag on the cat's collar. These special bowls are expensive, and I have no idea how well they work. This might be an option you could explore if necessary.