Introducing my 15 week old BA pullet to the flock

This is kind of a long story, but I'm hoping for specific advise, so here goes....

I hatched two Black Australorp chicks in late July - I started with 19 mail order eggs, 3 hatched, and 1 died a few days later. Since I wanted more laying hens, I bought 6 full sized pullets in mid August. Three of those have started laying, and have a well established pecking order.

Since September, I've had the BA babies in their own little run and dog kennel inside the larger run for the older hens. They have chicken wire separating them from the older birds, but they've been able to interact. One of the BA's started to crow a few weeks back, and since I live in the city and can't have a rooster, I re-homed him this week.

So, now I have one 15 week old BA pullet in the little run all by herself. She's about 3/4 of the size of bigger pullets and growing fast. She also has a crooked toe and a slight limp. I've let her out into the big run (supervised) once or twice. Things seemed to go OK, until the last time when all the older hens cornered her against the fence and started pecking her. Her instinct is to run away from them, but when she was cornered she couldn't.

So, my question is, what is the best way to introduce this young one to the flock? It's getting cold, and I don't want to keep her in the dog kennel for too much longer. I've read that it's good to give her places to hide from the older ones, but what's to stop them from following her to her hiding spots, cornering her, and pecking her to death? I'm especially worried since she does limp a little and I'm afraid she can't run as fast as the other chickens. This is turning into a stressfull situation - I really appreciate any advise!! Thanks!