I've had rabbits and chickens together for many years. Unless your chickens have coccidia, it shouldn't be a problem. Whenever I get chicks or have a chicken that I want separated, it goes into the rabbit colony shed. I have four broody banties, a auracana and a frizzle setting in the rabbit colony right now. They are all fine together. I just don't want some chickens to free range, so they go into the colony to keep safe. There are lots of myths about animals. Some people say you can't keep rabbits in colony at all, or have more than one buck in an area. It's hogwash.
I feed them all a cob mixture with some alfalfa added for the rabbits. I just use a basic grain/molasses/alfalfa pellet mixture for every animal from ducks to hens to goats to rabbits, etc.