I have them in together but you have to get them as chicks and only females, if you want to breed them then no. Certain breeds/personalities get along better than others, mainly the easy going ones. I have found I've had to give 1 duck away as she was causing trouble. I'm pretty sure that in a fight between adult chickens/ducks the duck would win but its never come to that. That being said, you have to make sure the hens lay their eggs up in a nesting box, not on the ground, as ducks are ALL feet, and will step on them and break them.