ducks have slightly different nutritional requirements than chickens. for one they require a bit more niacin, but there may be other things as well, so you should look into it. That said, ducks normally do fine, esp once grown, on a base of poultry ration. Though it also depends a lot on what ELSE they get to eat (hint: snails should be a decent source of niacin)--our ducks were free roaming almost 24/7, so i was less concerned about shortfalls in the poultry ration.
Whether to feed layer or an all flock ration is kind of a judgment call, or in some cases depends on what you can find. For example, the only affordable chicken feed we can get is layer, so thats what we use for a mixed flock of hens, roos, and adolescents (also ducks, in past). But again, we don't feed ONLY layer, they eat greens, veggies, scraps, bugs, and all sorts of other things too so the actual dose of calcium isn't as high as it would be if they ate only layer. If we could get a mixed flock type ration for a similar price, we would feed that, but we can't get it at all, much less for the same price. If you have a significant number of males or other non-layers, I would recommend going with the all flock if you can and providing a good supply of some good calcium source free-choice in a separate container (this is recommended even if you feed layer). If nothing else, it just seems more efficient: you'll get a little more nutrition for your money, because only the birds who need the extra calcium will eat it, and the ones who don't won't be working out their kidneys to poop the excess right back out.
Hope that helps...