You know, if you can't feed them two different kinds, I'd be more likely to keep the older cats on the younger cat food until they are all seniors, rather than the other way around. Unless it makes them fat, it probably won't hurt the seniors to be on junior food a little longer. There is not quite enough calorie count in the senior food for the younger kitties.
Here's an idea: mix the two foods together equally, that way all your cats get what they need. Ask your vet if this is a good idea. Blessings - Blu