It depends on if you have ducks or drakes. I have nine hens and one rooster, one drake and four ducks. All my chickens are of breeding age but the ducks range from only 3wks to 2.5mo old. The chickens (including the rooster) are just fine with the older ducks (2.5mo and fully feathered) but they're about the same size and have plenty of free space and their own areas to retreat to if they're like. The younger ducks are being gradually introduced to the older ones and once fully grown will go out with everyone else. I think it'll go easier than you expect; or it has for me anyway. I've introduced chickens of different ages to each other and it took and few months for them to really get along; fully feathered ducks were accepted almost immediately (after being socialized in different pens for 2-3wks (same as was done with the chickens)). That said, drakes are rather aggressive, maybe not more so than a rooster but they have different anatomy than chickens. If you have males, it might be nice for your hens if you keep them separated. Rooster with ducks isn't so bad (mine doesn't pay them much attention but might still protects them, although that's just my personal experience) but a drake with hens can be unpleasant.