It sounds to me like your August ducklings are now getting male hormones kicking in just like my two muscovy drakes. They have to work it out between themselves
My advice is that you keep an eye on the unknown variety that is being picked on the most. That duck is going to be bottom of the pecking order but shouldn't be injured as a result of that. Look out for feathers pulled from wings and tails and damage round the eyes. You would need to separate any duck/drake that is getting injured.
This happens with feral ducks too. A local wildlife rehabber and I just last weekend relocated a female pekin from where she had been abandoned over the Holidays on a pond at my library. That pond already had 6 pekins living there -- abandoned one by one over the previous year. Plus, recently 4 wild muskovy drakes had flown in. The newly abandoned duck was being really badly bullied and had lost her flight feathers and tail feathers. We had thought that she was a drake who was being bullied because he was competition with the other drakes, but it became obvious that she was female when the muscovies forced mating on her. We relocated her to a large pond on private land that has a wide variety of ducks living there -- domestic, wild, and migratory as well as geese. The land owner feeds the ducks in the morning and provides safe coops and stalls for brooding ducks to nest in. The bullied duck was accepted straight away and has settled down. I cannot rehome my bullied Pekin as he survived a raccoon attack as a duckling and has a damaged bill. I have to hand feed him from a deep narrow cup to help him get his food. He can chew on leaves on plants but has great difficulty picking up pellets or crumbles from the bowl in my coop. You may feel obliged to rehome yours if it is badly bullied, but that might shift the bullying to the next one up the pecking order. So its definitely best if the ducks can work it out between themselves
