Actually I'd get 2 that way if something was to happen [God forbid] you wouldn't be left with one duckling again. How old is Your duckling? And I'll share this 12 yrs ago I had a gosling hatch by my Muscovy duck, well I vent sexed and had myself convinced I had a female, evidently I didn't know...
Make sure you know what sexes you have because a drake will most likely when mature want to mate with a chicken hen and because anatomy is totally different a drake can kill a chicken if he mates her.
The 3 week old would probably work but I'd put them in separate brooders so they can see each other or in the same brooder with a divider so they can see each other just for a few days. To see how the Cayuga acts towards the little one. It should go fine after they have been together a few days...