Introducing female mice

Question
I saved a feeder mouse a week ago and named her Julia. Then on Thursday I got a fancy mouse and named her Agnes. Agnes is a little bit smaller than Julia.
They are like best friends in this one little tiny fish tank but when I put them together in the bigger one that I plan on them living in, they fight. Agnes chirps a lot and gets on her hind legs. But in the small tank they sleep on top of each other and are really cute.
No one is bleeding or has lost an eye, but I'm scared to leave them alone together in the big tank.

Is this ok? Are they actually getting along well and I'm just over protective?
This is my first time owning anything but dogs so I'm terrified of doing something wrong.

Answer
Dear Pressley,
thank you for your question.
Put them back into the small tank and leave them there for a day or so. Then they can move into a smaller cage or tank, but they shouldn't have any furniture or toys yet, just a bundle of hay to sleep and hide in. If they get along there as well for a few days, they can move into their permanent home, still just with the bundle of hay. If they don't fight, you can start adding furniture and toys one item at a time. Second levels should also be added later. I always start with some climbing toys and add the first hiding place after a week or so if everything goes well.
If they start fighting at one point, you need to go back a step. But usually female mice get along well, it's possible that you just put them into the big tank too fast.

I would recommend seeing a vet with them to make sure they are sexed correctly, especially since you are new to mice. Pet shop employees usually can't sex mice reliably. You can find rodent vets here:
http://www.rmca.org/Vets/
http://www.allaboutmice.co.uk/choosing-a-vet/
and these websites have all the info you need to take good care of your mice:
http://www.allaboutmice.co.uk/info
http://www.das-maeuseasyl.de/doku.php/en/species/fancy_mouse

I hope I was of some help to you
Jennifer