I had a Mallard pair and they mixed well for 8 months. However, we added two 8 week old pullets into the mix and they were smaller than the ducks. Everyone got a along fine and then one day we went out and the ducks had killed both of the pullets. We previously had a day old chick go missing from our broody hen one day too, but just thought it go out. We also had caught the duck on a partially hatch chick and it was squished. We thought it was just an accident.
It seems that as long as the chickens were bigger than the ducks, then they got along fine. The ducks went to my uncle's farm when they are with other ducks, protected, and free range during the day. I still go to visit them, but the chickens are much happier now.