How long have you been introing them? It might just take some time. Some of my rats didn't get along at all for a while, but eventually they got used to each other and now all live together happily ^_^
I'd say keep up with the introing process, maybe even starting it over. Put the cages nearby, switch cages, etc and just take it slowly. While you're doing that, start saving up for a bigger cage in case they really don't get along. Then possibly either get another rat for Marble to live with, or let Juniper live with Marble if you can't get another one (assuming Juniper is currently living with at least two other girls).
Juniper is living with my other 3 girls and they all get along ok. And I've done everything I can think of for intros (and what has been stated on this forum) but they don't want a new packmate.
I've been doing the intros for over a 2 months now and the other three girls just plain WONT except Marble... I'm getting a larger cage that Marble and Juniper can share but if they start to fight (which is possible) Marble will be in a nice big cage all alone.
If Marble is getting a big cage, instead of making her live in it alone, you should get another rat to live with. Even though she and Juniper may get along, I think that would be a better way to go. I don't really like the idea of breaking up an established group of rats who get along with each other, but that's just my personal opinion.
My female rats have generally accepted baby rats more readily than adults, so you may want to find a baby girl for Marble. And soon, since you'll have to quarantine her, and Marble has already been alone for months. No rat should have to live alone if there are other options, especially since she's not aggressive.
I'm going to have to get another rat for Marble to live with but my mom wont like it.... How old should the new rat be? Marbles about 4 monthes old so another girl just a little bit younger would be good?