Do some observing, try to find out who is doing the pecking. I do think space is probably your biggest issue. I don't see an obvious cockerel, but I will admit that I've been fooled before, different breeds can feather at different rates. Aggressive birds can be any breed really. Some people have trouble with RIR's, I've had more trouble with my wyandottes being testy, so watching to see what is going on is the best idea. Not knowing what the parental lines were, makes it hard to say 'that breed' is the problem. Every group of birds is different, what is adequate space for one group may be way to small for another, the individual personalities and temperaments are what really matters. Cramped quarters are the biggest cause of problems. I would really make the run larger, and make sure they have adequate space for roosting, sometimes the roosting bars are a battle ground in themselves. Make sure that they have places to hide behind (but that they have no way of being cornered behind). Sometimes out of sight/out of mind can help.