I assume all the other chickens have gone out, huh? I'm thinking of having you try something. Is there one in the older bunch that is particularly docile and seems to go with the flow more than the others? If you can try to find one of the older ones that wouldn't beat her up when you put them together in her cage, she would then have a buddy to bond with. This is a last resort sort of thing, and you might have to try several before you find one that will be a good fit. Any older one you place in her cage is going to probably peck her once or twice to establish rank right off the bat, so wait for that to get out of the way before you judge. What you want to see is the two acting normally together, not the bigger one shoving the small one around and terrorizing her. It could work out to where this chick will have the bond with another one that will help her in her acceptance into the flock, as well as help grow her self confidence. You won't know until you try it.