Sexing pekin ducks and duck pairs questions

I am trying to decide what ducks to keep what to sell. I have 2 pekins that are about 1 month old how do you sex them? I also have the same age Rouens but they are easy, I have 2 drakes and 3 hens. Is it best to keep them pairs like one male to each female, or would they do fine with say 1 male 2 females etc... I would like to have fertile eggs but don't need fighting between males either, do they do that? Seems like a silly question I guess. Anyhow my big question is how to sex the pekins, i will be keeping both of them but that will influence what sex of Rouens I keep.