FWIW, I have a small, dutch buck who lives happily with our 13 year-old Indian Runner duck.They are not free range (too many cats, hawks and wildlife here). They are housed in a shed with enclosed, covered pen area and they've been together since the rabbit came to us as a baby about 7 years ago. They seem to really like one another. The rabbit is quite calm, and easy to handle; I looked for that when we chose him from a group at the local fair. Also, over the years I've found our Dutch rabbits to be exceptionally good pets. We had one who lived to be 18 years old... he lived with ducks as well. The duck lays eggs constantly.
Obviously, a duck is not a chicken, but I thought you might be interested to know that with the right personalities, mixing it up a bit can work out just fine. If you try it with chickens, I'd keep a real close eye at first though, especially if there are several birds, and I'd make sure the rabbits have a safe place to escape and "hide." Don't think I'd try free ranging the rabbit under any circumstances... he/she would be a "sitting duck" for predators, so to speak.