I realize the origin of this thread is pretty old at this point, so my response is more for others' reference.
We've had 20 rabbits, 4 chickens and 2 ducks all together in a paddock (about 12x20 ft; decent size) for a while now. Behaviourally, everyone got along well. We kept the different families of rabbits separate to avoid the mothers attacking other's babies, but other than that they were all in together. The rabbits seemed to enjoy a good game of "chicken bowling". The male, in particular, would run laps around the paddock, build up some speed, then run right through the chickens. It was sort of entertaining watching all the chickens suddenly scatter into the air. The chickens go out during the day to free range in the garden. All the other creatures stay inside the paddock, unless we manually take them out and put them in a separate run.
All was well, until this morning, when our male rabbit suddenly died from some apparent bacterial infection. We didn't pay for the tests to determine exact cause, but after reading some of the comments here, I'm wondering if having the rabbits and the chickens in together TOO much isn't a recipe for disease. In particular, the rabbits were eating from the chicken feeder, and the chickens would occasionally get into the rabbit hutches when they were out in the paddock or in another run somewhere.
In retrospect, I'm realizing that this is an irresponsible setup, and we'll be making sure the chicken feed is accessible only to the chickens, and that the chickens cannot get into the rabbit hutches. Logically, this just seems like too much exposure to each others' poop and to bacteria transmitted from beaks and mouths.
A hard lesson learned.
With the chickens free-ranging and flying over the gate, it's difficult for us to keep them separated from the rabbits without having a separate and fully enclosed rabbit paddock. But that seems like the ideal setup.