I've heard that rabbits can be kept with chickens, but I don't think I would do it for a few reasons:
1. Rabbits have extremely powerful legs, and all it would take is one good kick to a chicken for a broken leg or wing, or worse.
2. Rabbits also have very large, very sharp teeth. They will use them if they get backed into a corner and feel threatened (I have unfortunate reason to know), and those teeth do a LOT of damage.
3. Chickens can do a fair bit of damage themselves, what with sharp beaks, powerful wings, and nails attached to strong feet.
The thing is, you might have them together forever and never have a problem (and there are probably people who have), but IF there is a problem, it's going to be a big expensive one.