I've sometimes used cat food for dog training treats.
Is it harmful? Mine are very large dogs and they have lived into their teens without any health issues besides getting a little arthritic. I doubt they get enough cat food to hurt them anyway. (a handful during a lesson at most)

I've also used dime sized slices of hot dogs and that's certainly not health food.

I have usually kept cat food and cat boxes either in a separate room or a hallway closet, with the door bungied or blocked just wide enough to be a 'squeeze' chute, too small for the dog to get through (but my dogs are usually rather large), or I've kept the cat's food on the washing machine in or some other shelf and the cat leaps up to get it there.

An extra cat litter box with narrow opening and a cover over the top, not used for litter but for the cat bowls can be handy.