I have 3 cats, all a very large fluffy indoor breed. They are 7 & 8 years old. One - the younger male - is obese at 6.4kgs, he doesn't run and can't jump. One is lean and fairly muscular at 5kgs and the girl cat is a petite 4kgs or less (she goes on steroids periodically, so it varies). Those two are playful, keen explorers and can and will jump on high surfaces - basically, normal cats.
The last time I brought Fat Cat to the vet I got a bit of a well-deserved lecture about his weight, and told I'd have to put him on a diet. I asked what to do when I have 2 other cats at home, and was told to feed him separately. I'm out of the house at work from 7.45am to 6pm and the cats currently have Royal Canin breed-specific dry food and water to graze on all day. But it's a very open plan downstairs and to feed FC separately I'd need to pen him into a small room like the utility or downstairs toilet all day while we are out. I do actually have the cat food and litter trays in that area at the moment and a stairgate to keep the DCs (age 1 and 4) out, but it is a small pen for Fat Cat without much to do (cat tree won't fit) and he can't jump the babygate or squeeze through it, so he sits and miaows pathetically until he's let through. I don't want to think of him sitting mournfully waiting for someone to release him from his prison for 10+ hours every day; that's not kind and he's not going to be any more active that way either.
I am not putting my normal cat food high up on the tables or kitchen counters (where FC can't jump); I don't want to encourage that behaviour from my non-fat cats.
If I change all of their food over to the diet food so he can still roam, would it be worse for the non-fat cats? Or would they do ok?