If they were adamant she become a feeder, there are probably a great deal of problems that are causing her to be as aggressive as she is.
She has probably been kept under very poor conditions with a huge amount of other rats, from which she has had to defend herself. I doubt very much that she has ever been handled for any length of time by a human being.
Genetics may also play a part. She has not been bred for her temperament, she has been bred to become snake food. You will probably run into problems with her in the future when it comes to her physical health. Buying from a pet store is already an unwise decision (one I have made myself), buying feeder rats is a worse one.
What's done is done, however, and you are stuck with her.
A breeder or someone who owns a rescue can probably help you better with this than I can, but when I first introduce myself to any rat I hold my hand next to the cage at a distance where I cannot be bitten, but I can be sniffed. You could do this every day for a certain amount of time, letting her get to know your smell. After that you could drop some treats into her cage to show her that hands bring good things, not bad things. Do not push treats in through the bars. If she gets the idea that things coming at the bars could be treats, she will be more likely to snap at fingers near the bars.
Keep her away from your other rats. She should be in a completely different air space (a different house, in your garage, a different room will not suffice), and do not come in contact with your rats for two to two and a half hours after you have been around her.
Whatever her temperament, she still needs exercise and care. You're going to have to find an open, rat-proofed place for her to run and jump around just like your other rats. Keeping her locked up is only going to make her angrier. An hour a day is the absolute minimum amount of freedom she should have.
It is going to take a long time. There are no short cuts for introductions between rats and other rats, or rats and humans. I have recently obtained a rat who is not aggressive, but simply nervous. She still cannot share a cage with my other rat. Don't get ahead of yourself. Take it slow and have patience, you can mess things up quickly by trying to rush.