I am having almost the same experience, and am desperate for a solution. We started with one hen house in a run of approx 40' x 25 feet. 5 beautful hens were left when we bought our house and we enjoyed them for a few years, then one by one they died of old age, between 9 - 11 years old. So when the first 2 died I got 3 ex-battery rescues and these 3 flourished, grew feathers etc., and we bought a second coup at this time to make sure the new girls didnt feel overwhelmed, but everyone got on fine. Then we got a further 3 ex-bats, and 2 cuckoo marans.
First the cuckoo marans started pecking the poor little ex-bats and our old original girls, The we lost all 3 old girls, one after the other with terrible wounds to ther vents, and their insides practically pulled out. We couldnt understand what was going on, so separated the new ex-bats to protect them. Then we saw the Marans chasing the ex-bats, pecking at their vents, with one ex-bat joining in the bullying. I read books, spoke to the vet, we had the Marans wings clipped again, but they continued to fly over a 4 foot fence and killed another ex-bat, so we took the offender to the Vet and had her put down. The last ex-bat now lives outside of the chicken run in the garden, and comes into the downstairs loo at night in a big cat carrier as the remaining Maran and ex-bat will kill her if I put her in the run. They try to peck her through the fence and have drawn blood as stupid "Lizzy" keeps running up to say hello.
Has anyone any clue why some of my chickens have turned canibal. They are great egg layers, but we really got them for sort of pets as well as egg layers. They have a huge run, the best food, we tried all the anti-pecking solutions, toys in the run etc etc., but I have to wait till they die now until I will even think of starting again, and even then having lost the most affectionate and gentle girls, is making me very nervous about starting again.