Ducklings are horribly messy. If you're set on one, I'd advise to get two and brood them seperate from the chicks. Brooding them together usually makes a damp environment for the chicks, and that's a danger to them, chicks need dry.
I'm not aware of a way to sex ducklings, and I agree having a solitary drake with your chicken hens could be quite bad for the hens. If you really want duck, I'd say look for a young adult or mature bird, so you know you're getting a female. You might also consider getting two, as they do better with their own kind.