I assume that your cochin is a full sized one? not a banty then. she would prob sulk a little but then be fine as long as she accepted aand they accepted her into the flock. it is a shame to have to separate chickens who love their buddies but sometimes it must be done. I relaise its hard but having a bully chicken is never a great idea and can lead to blood shed, I always try to keep the docile ones ( mine are generally docile anyways) unless I have to keep them because they are needed for breeding. admittedly my docile babies are treated better than their stroppy flighty counterparts as I spend more time with them and give them more treats ( not that my others are mistreated or anything I just don't spend as much time with them, which seems to suit them fine anyways)
getting back to the problem at hand , does your cochin get along with your bantys? if so then you could try putting her in with them to see what happens, if they get along leave them for a while then in a week or so try to introduce back the rir and see what happens, she maybecome more docile if she is no longer at the top of the order