advice on getting a new rat &introducing them?

Well, consider that she will not be around for much longer. If you get another young rat, it will end up being left all alone and you will have to deal with the intro process all over again. If you want to introduce any rat, get two so they will acompany each other later on in life.

If you decide to introduce, please please please, take the time to research how to do it right! Many people don't read up, don't understand what they are doing, and wind up with disastrous fights or worst. These are very easy to avoid, if you just take the time to learn about what to do to make it go smoothly. Intros are very easy, just do some research :)

First, females tend to bully babies, so you will want bigger girls. Take a look on petfinder, animal shelters, kijiji, craigslist, for people looking to rehome older females. You will need a second cage. You will need to do a quarantine period for 2-3 weeks, and introduce over neutral territory for at least a week. As long as you don't rush it, it will go fine!

It really depends on the nature of your pet rat. Since she has been with one rat most of her life it sounds, it will be difficult to successfully introduce her to new rats, but if you do it slowly it is a good idea. Get two girls, because that way when the older one dies off, you will not have to do another tiresome introduction to the remaining rat. Also, this will give your new rat a cage mate while you are waiting to successfully friend the rats to the older girl.

I have 6 rats. Only one of the girls is a fully grown adult. She is a bully to the male rats that live in a seperate cage. She will bite their toes very hard whenever they approach her cage. So you will have to be careful and watch their interactions very closely. If they do not seem to be getting along I would highly recommend keeping the new rat in a seperate cage and placing it directly next to the old rat's cage. This way she will get used to the smell and presence of the new rats :) Good luck!