Sorry to hear about the loss of your duck. I am guessing by white duck, you had Pekins. They are usually like 8-9 lbs. With a Pekin drake, they are quite large and can be too big for smaller females. Mixing breeds is otherwise not a problem. I have Saxony, welshies, khaki, Cayuga, Pekin, swedish, runners, Muscovy and they all get along fine.
Your biggest worry is going to be the chickens right now. Drakes have a penis which chickens are not designed for. Without a female drakes will mate chickens and can hurt or even kill chickens on accident.