My part wild rat ate mice, she got all freaked out when she saw them and attacked like a crazed wolf. Fuzzy Rat was adopted at about the same time as my daughter had a litter of baby mice and we put them together young and they pretty much ignored each other. When I had the mice out in a play pen for them, Fuzzy Rat jumped in and stole their food but otherwise never reacted to them, it was like she didn't see the mice and they didn't see her. Like rats and cats, I suspect rats and mice can be friends. Replicate this at your own peril. Getting another rat can very well be the end of your mice unless you resolve the communal living situation. A new rat might see your mice as tasty treats and one quick rat bite and your mouse is history.
I suppose it might make for an interesting experiment whether your rat might be happy with mice as friends long term, but then they would have to actually be housed together so they can be free to interact 24 x 7. But keep in mind, mice are generally far less competent than rats and really can't interact with rats on the same level. It might not be very satisfying for your rat in the long run and can go terribly wrong, so despite my curiosity, it's an experiment I can't encourage in good conscience.
That said, two rats are actually easier to keep than one. I know this sounds counter-intuitive. But rats will play together, and keep each other company and that gives you more free time to do other things. I've raised an only rat, and we provided human companionship 24 x 7. So our rat went shopping with us and traveled with us and as someone was home all day hung out with us... Once we had two rats, I could finally leave our rat at home sometimes and even take the family to a restaurant that didn't allow rats. A second rat actually made life easier in a big way.
We currently have two rats, Max and Cloud. Max is house safe and lives in a metal cabinet from which she can free range the house, Cloud gets bored and sometimes destroys stuff if left out overnight so she has to go back to the cage when we go to sleep. Last night when I called Cloud out to go to the cage Max came out of the nest first and pushed Cloud back into her nest and blocked me from reaching in to get Cloud out. There's no debate about it, Max didn't want Cloud to go back to the cage, she really wanted to stay together with her friend. Well, I offered to let her sleep in the cage too, but she declined. She's earned her freedom and she's not going to give it up for anybody, we'll see if this changes come winter and that cabinet gets really cold. When Cloud was injured she ran straight to Max an Max cared for her, they have a very special bond. Yes both rats are bonded to us and enjoy being and playing with us, there's no denying that they love to be together too. So unless you can be with your rat around the clock every day, she would be better off with a rat friend.
Lastly, I don't mean to imply that mice are stupid... They are pretty intelligent too. But I've got to imagine the best relationship that's possible between rats and mice is like a 15 year old girl and her 6 year old brother. Sure it's better than nothing but it's not like having a best friend her own age. Rats and dogs can even be best friends, but I have lots of trouble imagining a rat keeping up with a dog playing Frisbee at the park and my rats do go to the park with us. Your situation might "be working", loosely defined, but separate cages and a rat friend for your rat would actually work better for everyone.
Best luck.