Hello Jenni,
chickens and ducks should get on fine together once used to eachother. I kept all sorts together and had very few upsets. As for cross breeding, they are different species so no problems on that score at all . Ducks will need plenty of water , so keep an eye on any young chickens in case they fall in !!! I just used lots of shallow bowls with water about 4"-6" deep and never had one problem . The only duck to cause any problems was a big old muscovy , he would go for anything that got near him . All the other ducks and chickens just kept out of his way ....as did I ...I kept ducks , geese , chickens , guinea fowl and even a few pheasants that got out of pens , doves and for a while peacocks. They all managed to get on ok , so you should be ok . As with any animals ,when you get new ones introduce them carefully and keep an eye on them for a few days. Hope thats of some help...
cheers
Russ