I wondered about putting our new 4-H rabbit in the coop as well, until doing a little research! Rabbits can catch diseases from the chickens that the chickens are immune to (such as cocci). Another thing is that a rabbit would get so filthy with the chicken poop, dust, etc from the coop. Plus, the rabbit may get pecked or injured and that is just so unfair for the rabbit. Their only defense is running away and hiding, which would be hard to do in an inclosed area.
I ended up building our Dutch rabbit a hutch, near the chicken yard. However, "Dutchess" now resides in a large cage on top of my son's dresser (living like a queen) until spring! It is a better situation for us because my son can work with her daily on showmanship and is easily reminded to clean out her cage!
I hope all your animals (chickens and bunny) will be happy with their living arrangements.