This isn't necessarily the right way to go about it, but this is how I introduced my rescue to my pair:
I kept them all in the same room, not near each other but they could see, smell, and hear each other across the room through their cages. I left them separated for a month or so. I was worried because one of my pair was aggressive when in a larger group in his previous home, and the new guy had been a single glider since he was 6 months old. I was also waiting to get the rescue neutered before attempting intros. The rescue would bark to them every night.
I would sometimes let one or the other roam the room while supervising, and they would invariably end up sniffing each other through the cage bars. At first I would stop them since I was worried about toes getting bitten off and the like, but I paid attention and the encounters were always curious and not aggressive. I moved their cages closer after that so they could see each other better as well.
After the new guy was neutered and had recovered, I swapped his pouch with the one in the pair's cage so they had to sleep with each other's scents. I would leave them for a few days and then swap again.
After a week or two of pouch swaps (I don't remember exactly how long), I decided to try letting the rescue and the less aggressive of the pair meet in the bathtub, since the resuce was lonely and they were very interested in each other. They immediately crawled in the pouch together and he grabbed onto the rescue's back like a baby. I brought the more aggressive one in and he crawled in the pouch too and rubbed his chin scent gland on the rescue's head, claiming him as part of his colony. I slept by their cage to make sure no fights happened and they didn't.
It sounds like the female just needs more time. I would just let them be separated for now. It's also possible that she's picking up some of your stress as well, animals are sensitive like that. Remember that you are giving them a loving home and if you are just feeding them and not interacting with them too much until she's more comfortable that's fine and what she needs right now. I would make sure to feed them extra as well, since she's underweight. You could try putting them in a separate room from the others but if her behavior doesn't change then I'd put her back in the same room, not close but where they can see, smell and hear each other across the room. I wouldn't start pouch swapping until she is at a normal weight and comfortable with you.