Hen attacked and injured when introduced to flock. Suggestions?

Trying to introduce a lone hen to an established flock is not easy. Normally you should quarentine a new bird to check for illness before even trying to introduce them, but since she's already been with them that's a moot point now. You'll have to keep her isolated from them until she heals up or they'll just keep beating on her and pecking the wounds. If you can keep her where they can all see each other without any contact after a few weeks you can try to reintroduce her.

I'd also try to rehome 2 more of your roos. 3 roos on 3 hens is too many roos.

 

Here's what I did when I had a broody hen to reintroduce to the flock (plus she was the bottom of the pecking order, anyway). I picked another chicken that I knew she got along with (they were clutch mates) and put her in a separate pen with the former broody (Hermione). Then after a couple of days, I put another low ranking chicken in with Hermione and her buddy. A few days after that, I added another chicken to that trio. I kept this up until the whole flock was now in the second pen.

When the higher ranking chickens joined the separate pen, they joined as singles and were being added to a new territory, so they were a little off balance. They did not whup on Hermione as much as they would have if she had joined them on their own territory. Yes, Hermione did get picked on some, but not as bad as if she'd had to go through the whole flock picking on her in a single day. She did not get ganged up on.

By the way, I've never heard about putting pepper in an open wound. That sounds painful! Personally, I'd use flour or cornstarch to stop bleeding, if necessary.