As a rabbit breeder, I can tell you that letting your rabbits run loose in your barn is not a good idea. As everyone else said, they can get coccidia. Besides that, letting roam around in a building like that can be dangerous for them. They are also very destructive when it comes to chewing. if your building is wood, they will chew their way out. Best thing to do is keep them caged, in rabbit hutches, seperate from the chickens. Rabbits have different nutritional needs then chickens do. some types of chieck feed(the medicated stuff) can be toxic to them. Also, a caged rabbit, fed rabbit pellets and hay will tend to finish out better, and be in better condition then one that isn't. You also take chances of them breeding like crazy when the run loose together. Then you end up being overrun with a bunch of severely inbred rabbits.