A single hen is really one of the worst integrations. Everyone in the established flock knows she is a stranger, and might eat all their food. Everyone wants to peck her to make sure she knows her place.
But all is not lost, there are tricks you can try. If at all possible, pick a middle hen in the current flock, and put her and the new hen somewhere else for a good week. Now these too probably won't get along at first either, but it is one on one, so should be fine, just a few skirmishes as they work it out. Then introduce the pair back to the flock. After 10 days, the flock will have forgotten the original hen and be positive that they have two new strangers to eat up all their food, but even a mean hen can only peck so much, and with two new birds, it kind of spreads the pecking out.
Setting up multiple feed stations also help, and better is a little wall such as a piece of plywood, so that birds can get behind it and out of sight of the other birds. If they can eat there, where out of sight of the other feed bowl, so much the better.
Do take a look around your run, is it just an open rectangle, with only the ground level available to your birds? Add multiple levels, pallets up on bricks, extra roosts, all make the run more interesting, and increases the use of all dimensions of the run. I may make it look a bit cluttered, but it adds usable space to your set up, lets chickens get away from each other and out of sight.
Good luck, these are some ideas, but really if you do nothing, ten to one, in a few days, they will be over it. As long as they are not drawing blood, leave them alone is very good advice.
Mrs K
