We have two rabbits that started in a hutch outside but quickly inherited a large area on the side of our house to run free in - about 450sf or so. As long as they have plenty of timothy hay and their pellets, along with occasional treats, they are fine there. There are times (like tonight) where they had the run of the yard for several hours, but they are fenced in and we can see them.
One thing I will say is this: Of our two rabbits, the dominant (confidant) one will wander back and forth between the yard and their area if given the chance. The other, once out, does NOT want to go back in (probably because the first one picks on him a lot), and would rather stay under the bushes and foage around the yard than ever go back to their food and water. It was a real challenge just getting him back in the gate tonight.
I hope to introduce some hens to the rabbit area soon, though the hens would be the ones in a run while the rabbits live semi-free. Eventually i'd like them to be able to intermingle, but not for more than an afternoon. My concern is more with the chickens making an even bigger poopy mess of the area than the rabbits have.
I've read what little I could find on the topic, and other than the occasional bad experience where the hens peck the rabbits, most seem to be ok. The way mine run, I can't imagine a hen keeping up as they run through tiny openings and between things.
-Dave