Again... there are MANY types of mites and some are microscopic. You will NOT be able to see them! So unless you have had a skin scrape done, I don't know if you can tell FOR SURE that you don't have them.
If it is a learned behavior, sometimes it's because the diet is too low in protein. What do you feed including supplements?
Peepers can be used to help stop feather pecking. I would be saddened to!
As well, I would see WHO doesn't have any pecked feathers and consider quarantining if not eliminating from the flock. Maybe put some Blu Kote on there or some bitter agent to stop the pecking. Since red attracts pecking it can be inviting it even more.
Also, some mites that CAN be seen don't live ON the birds. They live under the roost or in cracks of the wood and only come out to feed at night. So you wouldn't find them on your birds unless you were checking after dark.
But yours really does look like bullying since you don't have a rooster.