were they mature hens or fully grown pullets that you introduced? Were the RIR's all from one flock? Generally all around here will recommend a 30 day quarantine period before introducing outside birds to your existing flock. Disease can take up to 30 days to show
As for solving the current issue, there are lots of things you can do
1.) Separate the pecked bird but keep her in the coop, partition off an existing area if possible, that way they cannot peck her but can still see/smell/interact with her. Put their food and water bowls next to each other with some kind of see through partition (chicken wire), that way they can eat and drink together while protecting the pecked bird
2.) Dust her with some anti peck dust - you can get it at a local feed store, it tastes terrible and the chickens will stop pecking her
3.) If there is blood, get her out of the coop - red is irresistable to chickens, they will peck, peck, peck until she is dead if there is blood showing. Put some red toe nail polish on and go into the coop with flip flops! Then slowly re introduce her after the wound is healed and the feathers grown back, do it with plenty of space and some time so you can observe the flock. Put a ring around the offending RIR so you can check she is not the only one up to the pecking tricks
4.) Take out the menacing bird and rooster from the coop, let the others establish the pecking order and then re introduce them as the minority, they should fall into place - rooster probably at the top of the list the hen elsewhere
5.) Let them free range together if possible, if they go after the bird she will have space to escape!
6.) Finally if you observe pecking in person, i kick the offending bird, not a boot like you would a football, more of a scoop, enough to ruffle the feathers but never to hurt them, it lifts them 2 - 3 feet off the ground, they stomp round petulantly afterwards but after a while they get the message
Ultimately you may not be able to stop it, i have 3 RIR's, they bow to my every (almost) command, i have 1 mille fleur bantam, they torture her and peck her for no reason, my coop is big enough that she can escape, she quit roosting with them a few weeks back, she roosts high up on the roof joists, away from the others, i have a run, she escapes the others, eats after they eat, drink after they drink, scratches, dust bathes and generally everything after the RIR's have had their turn. Such is the life of a flock of birds and their pecking order!
Good luck and remember as much as they are pets and i do see mine as pets that contribute to the families bottom line, we cannot project human emotions or behaviours on to them, they have their instincts and sometimes are set in their ways!
