Welcome to the party.
Let's say the chicks are pretty much grown up and now you want to get some ducks. Just raise the ducks separately until they are about eyeball to eyeball with bigger chickens. Then put them together and make a fence or barricade between them for awhile. The point is to let the birds get used to each other. They need to be able to see each other but not touch. Feed them around the barricade or fence, feeding birds together makes them more calm and used to the other birds.
Then let them join. They probably will not be flock buddies, ducks going over here and chickens going somewhere else. The ducks will most likely be above the chicks in the pecking order. I've seen a 3 pound Khaki Campbell hen run right into a 5 pound full grown rooster and make him leave the food tray. Ducks are very strong, they dont care. My main Khaki drake will even nip at the African goose. Nobody gets hurt, feathers aren't flying, ducks are just tough little packages. They run about 107 degree temp and dont usually get sick.