Tips for trust with previously abused and skittish rattie?

We recently adopted a rat from someone who couldn't handle her because she was always biting them and fighting with her other rats, so she was kept isolated for a few months in a cat carrier. The old owner said she wasn't sure where she came from initially but thought it was a horder situation. That does fit with temperament, but she also seems to have a fear of hands

Sometimes she's really sweet and will let you pet her from a distance if you are really slow and she isn't already spooked. But whenever something gets her nervous (and many ratties get nervous over anything) she will bite hard and won't let anyone or anything near her. She prefers a small hamster cage over being in the large cage with my other 2 (or at all, as it seems to get her worked up), but I've been keeping it right next to it hoping eventually they will get along and she'll be willing to integrate. The others are happy to help, but she just won't let them yet.

I know it will take time. This isn't my first hard case, and I'm willing and certain I can rehabilitate her. She has a good rattie personality otherwise, but gets scared easily and her first instinct is to bite hard and often. I'm willing to work with her as long as it takes, as I have with two others in the past that had similar temperaments (one never got past it and one became the sweetest little girl ever) but if anyone has some tips to help, I'd love to hear them.