I house chickens and ducks together. They all eat Purina Flock Raiser, which can also be given to turkeys and geese. I would say that space is a big factor. My coop is well over 350 sq ft with 30 birds (I do plan to increase size of flock). My birds also free range on a couple of acres all day. Even during bad weather, when no one wants to go out, there is plenty of room for them to get away from each other. I also have straw bails and a pallet that blocks the view so they can get out of eyesight.
There is an occasional bop on the butt but nothing serious. It's never been anything more than normal pecking order type stuff. I'd say most times the chickens are rougher with each other than anything that goes on between species. Though the worst was one cockerel who would not give the ducks their space. It took him awhile to learn his place but there were no injuries, mostly just pushing.