Aggression between my drakes can be pretty hard to watch. The girls are more like the school yard bully - pestering, nipping, chasing without intent to catch. As long as no one gets hurt, they will need to work out their own pecking order.
I was glad to see that I'm not the only one thinking this. I was on the verge of posting this question.
Love your set up with those flat rocks. I'm going to copy that in part of the run next spring. I bet the duckies would be very interested in looking for worms if I occasionally turned those...