That's what I've observed in my flock. ..the lower they are in the pecking order the harder they are on the newcombers. I'm not sure if it would work for chickens but I've read of people who are trying to get their pet rabbits to bond will put them in a carrier together and take them for a car ride because putting them in a safe but stressful situation gets them to focus on what's happening other than fighting with anyone and they soon get over the newness. Your off to a good startby ggiving them the free-ranging time. The more space they all have the less threatened they'll feel
I have noticed that also. The bottom of the pecking order here had blood on her back from pecking. We put her in the hospital for a week or so. The other birds would visit her, but when it was time to be released the bottom birds were relentless on her. She is a smaller timid white EE, and spend most her day running from the others. She is even afraid to enter the coop at night. I have to wait till she roosts somewhere and carry her into the coop, and place her on a high deserted roost area.
My birds have 100's of acres to use, and still this one is picked on. The sad part is I like her and I think I will keep her for my flock. When the rest are departing she and 12 others will get to stay.