Also do not be alarmed once you find a Hen to add. Their will be chest bumping and neck tying in order to establish a rank in the flock. The best way to introduce Ducks is to free range and let them figure it out. Might look bad but its really not...
It all depends....In general it is easier to add Ducks the same age. Some Ducks do not accept young Ducks that are still peeping...I tried doing it and it seemed to go well the first three days till my one hen grabbed the little bugger and swung it by its neck...I sold the two little Ducklings...