Feather sexing on new hatchlings only works on specifically bred crosses, and since you don't know the parentage of your chicks to be, it won't work. You'll have to wait until the chicks are 6-8 weeks to know which are cockerels and which are pullets.
If you're asking will drakes rape hens and pullets, the answer is yes. You can wind up with dead/horribly injured chickens. Ducks have penises, chickens do not.
If you're asking "can I make duck/chicken hybrids?" the answer is no.
Good for you to recognize that you need to process your birds before they kill each other. It doesn't mean it'll be easy, but you have made a good decision. Your jenny/young hen will thank you, too.
Fasting the birds makes it easier to clean their carcasses, but it's not 100% necessary either...