I have 2 groups of hens, three are a year old, three are 6 months old. I REALLY wanted them all to "coop" together, but 2 of the youngest just can't handle it, they are just TOO stressed. Even though they are very close to equal size, I have them housed separately (2 & 4), but all six forage together. The one young'un that I did put in with the older ladies, is lowest in the pecking order, but although they bully her a bit, she's doing okay. She eats, she lays, she can't wait to get out, away from the others, but she isn't getting injured or anything. They all do fine out in the yard. They kind of break into two groups- older/ younger, but there is peace. I just have to make sure that they have plenty of time to get away from each other.
On another note, I did have my youngest Buff Orp get injured- not sure if it was a cell mate, or something else. She is absolute lowest on the pecking order out of the six. I brought her inside for about 2 weeks, then I put her out. She actually did better afterwards. She had a chance to be #1 while she was in the house, and gained some weight, and her confidence improved after that.
I wouldn't throw the towel in on them yet. Make sure there are not any bloody exposed spots, and give them EXTRA care. They might grow bigger than the rest and then fit in just perfectly afterwards.