I don't think the rabbit droppings would harm the chickens at all. They are very good fertilizer, you don't even need to age it like you do for chicken/cow/horse poop. Had an aquaintence that raised rabbits for meat, and his garden was spectacular.
I believe rabbits have two different poops - one that is the hard dark brown one, and one that is softer. The soft ones are actually good for them and they eat them. Now I have to go look it up on the web...