I agree that the dynamics can change dramatically by removing the other birds. Unfortunately for me, it went the wrong way. When I first started out, I had two and kept both birds but split the flock in completely separate locations as they were fighting. The dominant cock who had never been a problem to me before I separated them, suddenly decided that since he had no completion from below him in the pecking order, he would challenge me. We had several run ins before he accepted that I was in charge and he was my deputy. The other cock, that had been submissive to him, became more aggressive towards me once he had his own flock and we had almost daily run ins. I was unable to train him or even negotiate much of a truce, but he eventually gave his life in defence of his girls against a dog, so I suppose his "big man" attitude served a purpose in the end.
I won't tolerate that sort of behaviour now, but thankfully having an established flock and youngsters growing up within that, means that they learn not to push their luck from the older hens and head cock and I haven't had any problems since, even though all my birds are bred from those two original cocks which would probably be considered " aggressive"..
