what type of comb does she have? Do you know her age? She looks EE, but she could have game in her? I'm not as familiar with the game birds.
Its always difficult to integrate a single bird into a flock. There's only one 'target' for the established flock's territorial aggression. And that's what this is. The current flock is defending it's territory from what they see as an interloper. That's why this little lady is hanging out with you. She sees you guys as safe, since your not picking on her.
If you have the room and the ability, I'd get a couple more pullets, from different places (so there's no relationships). I'd put those three (including this one) together in their own pen, so that they could befriend each other. Once they have bonded and become their own flock, then I would start integrating them with the rest of your flock. With the multiple 'targets' there will be less picking for each bird.
