Once they start, it is nearly impossible to stop. Your set up will fit between 6 to 7 full sized birds much better. Because you have this problem started, it might be better to reduce to 5 birds. I would imagine that there is a lot of tension in your flock.
This happened to me once, I got bit by chicken math, and more is better, but a predator got in, and took out 3 birds, at first I was mad, and fixed the run to prevent it again. However within days, I realized how much more peaceful my flock was with the less number of birds. A peaceful flock that fits into your set up is a much healthier flock.
What do you have in the run? Is it just a big open space, and all the birds must stand on the ground? Adding some pallets, some up on blocks high enough that birds can get under, and stable enough that birds can get on top, lean another pallet against a pos, but leave both ends opened so as not to trap birds, thist will allow birds to get away from each other and out of sight of each other.
While making the run look rathered cluttered adding these can add square feet to your run, as in you have the area under the pallet + the area above the pallet. Put some roost up in the run in the corner. These give the birds more space, some exercise, some interest and some retreat.
However, I agree with the others, cut the flock in half, and add stuff to your run. First I would get rid of the two being picked on, and the worst one for the pecking, and then see.
While it can be diet, most problems are caused by over crowding, in commercial places, this is why they are debeaked.
Mrs K