Baby rabbits stress really easily. Rehoming is a huge stress for any rabbit, but if it is done to a really young bunny, the stress may be enough to kill it. For this reason, in many places (my state, for example) it is illegal to sell a rabbit that is less than 8 weeks old.
In the "circle of life," a rabbit is a small herbivore. When you are a small herbivore, it seems like everything eats you, so you need to be scared of everything if you want to survive. Rabbits need places to hide, places where they feel secure - chances are good that the first time you turn your rabbit out to 'free range' will be the last time you get your hands on it, maybe even the last time you see it. Some people have rabbits running loose, but only because there are virtually no predators where they live. I have a friend who tried to raise rabbits on the ground; she lost rabbits to a variety of predators, including hawks, owls, snakes, foxes, dogs (including her own), and drowning when their burrows flooded in rainstorms.
It's really hard to say how chickens and rabbits will get along. Quite a few people have tried it; some have had chickens that attacked the rabbit, some have had rabbits that attacked their chickens, some rabbits and chickens just pretty much ignored each other. The biggest issue is feeding - while rabbit food probably won't do a chicken any harm, a rabbit shouldn't be allowed to get into chicken food. Besides the fact that a lot of chicken food contains meat by-products, the nutrient balance is way off, particularly in regards to carbohydrate content. Feeding the wrong stuff, or even too much of the right stuff, just fed before the rabbit's digestive system has had time to get used to it, can cause diarrhea, which can kill a rabbit quickly.
