It's so hard with these little guys as just like us they all have such unique personalities. Poor Fred does look weary in that photo. The one time my Sally got a couple of wounds on her back which appeared to be from someone pulling her hair out, or an accident or dry skin that she had scratched my vet shaved her around the wound (although was just on one site) and made it much easier to keep clean and monitor.
I agree with you that it is very hard to know when a line has been crossed and need to separate. I've had 3 piggys who suffered a sore patch right in the middle of the back. One I eventually blamed on dry skin from bedding as was using shavings at time (never happened again after switched), one I am certain was caused by being chased through one of those wooden willow sticks that you can curve into a tunnel and catching her hair, pulling it out. Then the last was Scrumpy a few weeks back. I suspect that may have been a spat between the boys as that is my male trio but it healed quickly and nothing else since. In all those cases I didn't separate and the groups have had no issues since but you gave them a chance and then had no choice but to separate when you saw a fight break out. You didn't see the plucking occuring before and had no way of knowing that was what was happening. It's hard to know what to do for the best some times and I think you always acted in your boys best interest and don't forget that it's easy to see things differently with hindsight. From what I know pulling hair out on a regular basis as bullying is no where near as common as pigs (especially males) getting into an all out huge chunk biting fight so it's just not something I think many of us would be looking out for. Perhaps it is all in what is the definition of a bite and with hair plucking rather than flesh biting it's knowing when to recognise it and seeing an escalating pattern occur?
I hope Fred heals quickly and glad to read that he is already popcorning and chatting. I've heard so many good things about Cerys and certain she will find Marvin a good and suitable home. I know they do homechecks as well as I've visited a couple of times but was told too far out of their area for a homecheck.