It's usually fine to mix chickens and rabbits, but not always. It is possible for them to injure each other, since rabbits are not accustomed to being pecked and chickens are not accustomed to being kicked. If either happens, you could have bloody animals. So I recommend a similar introduction period that you would use for introducing same-species animals, to take some of the fascination out of it and make seeing each other more ordinary.
It's also possible for rabbits to get sick if they eat chicken droppings. The other way around is fine - chickens can peck through rabbit droppings with no problem at all. If there is a way to minimize the amount of food that is loose on the ground, it will help. And if there is a way to keep the chickens from pooping in the rabbit food, that will be helpful too.
Of course if you keep the rabbits in their own hutches, just in the chicken area, then that's just fine. No injuries, and the droppings will go only in one (safe) direction. But mixing them on the ground often works OK too, if you take a few precautions.
Hope that helps!