My breeder allowed me to choose my own ducklings, without sexing them, the first time. I chose four, and as they grew, it was obvious three were drakes (She never again allowed me to pick without her "approval"; she's only misidentified once). Maybe because they all grew up together, the three boys didn't fight with each other, and although Quinn got a lot of romantic attention, she didn't seem to mind.
In fact, when I brought in two more adult runners, Quinn seemed upset when the boys paid attention to them. My adult pen has three drakes and four females right now. I have a pen of four more girls who will be fully integrated by winter, I hope. They free range together during the day, and none of the drakes have shown the slightest interest in the youngsters -- or the adult girls right now. It's one of the few good things about winter here!