I'm not saying this will work for everyone, but here's my experience with runners.
When I chose my first four unsexed runner ducklings from the breeder, I ended up with three of the four being drakes. Because I'd heard that could be catastrophic for the lone female, as soon as I realized the potential problem, I went back to the breeder, who only had one girl of the right age that she would part with.
However, the original female seemed jealous of any attention the new girl got from the boys and would physically get between her suitors and her new "rival." And, the boys, having grown up with each other, rarely got into squabbles with each other. At most, there would be a bit of head batting, but no real injuries.
So, MAYBE things will work out on their own. Or, you could rig up separate quarters for the boys and girls. In past years, when a newer drake shook up the social order, that what I've had to do.
BTW, I love mine as pets, and they are good layers -- although they do take a winter break.
Good luck with yours!