Well, not a week has gone by and Buffy has been isolated from the other 4 hens at night. She has come to know where her new resting place at night is. I let her out most all day to free range around the yard and if the others are out with her she usually hides. If I keep Cadbury in the pen she will mingle with the others but everything changes once I let Cadbury loose. I've noticed in the morning Buffy has what looks like course dry skin on her back, visible on the outer feathers and I am beginning to see more and more that her feathers on her chest seem scarce. There has not been an attack in awhile to warrant additional feather loss.
So is Buffy's poop normal now? She is eating and drinking well? Perhaps the "off" poop was just a one day thing? If Buffy is healthy and she was the original bully to Cadbury, hens keep account of transgressions. So, in this case (assuming Buffy is healthy) Cadbury has risen in the pecking order and is paying Buffy back for all she did to Cadbury before. Buffy must now submit to being lower in the order. They may never get along that well. When you introduce the new birds, the order will change again and maybe, Buffy will not be as prominent on Cadbury's radar. If Buffy is losing feathers in front, she may be getting them pulled out on the roost, pulling them out herself if she is nervous or beginning a molt. Buffy's molt (if that's what it is) could make her isolate herself as molting is uncomfortable but, from your pic, if she is molting, it's a slow/soft molt where she would not separate herself. You really need to spend a day watching your flock interact and what is going on at roost time, it's the only way to figure out what is going on. If Buffy is not accepting a lower rank and doing things like eating/drinking at the same time as Cadbury or not giving Cadbury a wide birth then Cadbury will keep physically dominating her until she learns/gives in. If Buffy is healthy, cadbury will not be able to kill her with out help from other flock mates. So, hows it going with them?