I have rehomed a few more aggressive hens when they were causing trouble in the flock. I have never used pinless peepers, although many people on BYC have used them successfully. I would use them on just a bully, since they only prevent the hen from taking aim to peck another. Otherwise they can eat and drink and live normally. A lot is up to you. Separating the bully for a week or two sometimes works, and the others can adjust their pecking order. @azygous has a good article on how to deal with bullies and how to restore confidence in an underdog. I will try to find it.