When you mix species, the trick is to feed to the highest nutrition requirement, which in this case, is the ducks. They need more protein & vitamins (think about what goes into building that very heavy coat...) than the chickens do. I keep ducks & chickens together and they co-habitate fine...
In NC it's only $5, along with the form & you have to agree to follow certain rules... My Jumbo x Giant Bobwhite are WONDERFUL to have around! Smart--my Breeders know I'm not going to hurt them & they allow me to: drive the lawnmower right past them, without spooking; reach in right next to...
Saxonys! Very smart, eat LOTS of bugs, lay 220-240 HUGE eggs/year, beautiful (and DON'T look like they have Cancer...). Large enough to be great table ducks. You can tell the sexes apart, too, by markings & bill color (even as little ducklings), which is often a problem with Pekins...