DONT put your rabbit in with your chickens. It will get sick. My friend got a mini Rex last year and kept it in the chicken coop with the chickens, but it had it's own cage. It died a few days later. They called the vet and the vet said that rabbits can get some sort of disease from chickens. I don't remember what it's called, but its deadly. Please don't do it.
My rabbit has been with my chickens for 2 years. The rabbit is in it's own little yard and house, but the chicken yard surrounds the rabbit. The rabbit is never loose with the chickens, but they are always near each other. Could it be if the rabbit gets in the chicken poop?
Rabbits have a very peculiar digestive system and don't tolerate some types of food. Best not to let them eat chicken crumble. I know of abreeder who had some rabbits die and he was convinced it was because the feed was contaiminated with chicken feed by accident. I also have been told to keep my rabbits away from chickens.
Gosh, I have my bunnies in the same run (not same coop and hutches) with my chickens and thye haven't had a problem. I feed their specific foods in their coop/hutches, and food either can have in the run. www.hencam.com was my inspiration, and I have a similar set up. Wait till daytime and you can see his or her set up.
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.
putting a rabbit is with your chicken have be dangerous, The chicken could pick on the rabbit. Most the times when people are able to keep chicken with rabbits it's cus they grew up together since they where small.