It really depends on how many females you have. I have never had a problem with this. I had 2 drakes and 6 girls and they got along great. How many ducks are you going to get? I would say to have at least 2 girls per drake.
Keep the drakes apart at night, that has helped my flocked immensely. All the birds daytime free range together, my ratios are skewed but again i did this change and it settled things way down. Personality, of the drakes does make a difference too. Always best to have many ducks to drakes... you can never go wrong with that set-up./