I have three drakes and one hen, and haven't had a problem with them being too aggressive, which is funny, because I totally thought I would have to separate the female with a specific male. She is a Cayuga, one drake is a Khaki Campbell, and two are White Pekins. Curly, one of my white pekins, never quite manages to mount her (his neck is curly--thus the name) and I expected almost all of my babies to be park Kahaki because she runs with him the most, but they appear to be almost all white pekin--so Shep has been a busy boy!
My ducks and the geese I raised them with from babyhood all think they are siblings--the drakes try to mate with the female geese, each other, and the gander tries to mate with everyone. My Muscovies were added later, and I've since taken to putting them into the chicken pen at night once the chickens are closed in so they don't get picked on by the others. They all run together during the day.
You can probably let them all run together until mating season, then split them by breed and weed out extra boys if needed. (Unless you have a breed that lays several nests a season like Muscovies usually do, but even then you won't have to worry about it until late next winter). This will give you extra time to decide which ones you want to breed.
Next year I might separate Larry and Moe from the rest of the ducks and see what adorable babies I get! (Yes, my ducks are Larry, Moe, Curly and Shep--and Larry is a girl, I had to name them somehow, and I had no idea who was what gender then.)
