Relocating mom and baby mice to new cage and housing male mice together.

Question
I brought 3 mice at the pet store almost 3 weeks ago. Sophie, Kiki and Mizuki. Sophie turned out to be heavily pregnant, so I put her in a small, but suiting nursery tank. She had 9 babies and they are now 12 days old.
Later on, I found out Kiki and possibly Mizuki is also pregnant. So I brought a 5 gallon tank for Sophie and her babies, then I will put Kiki (The most pregnant one) in the nursery cage. Mizuki is going to be in the cage until I get another tank.
The only problem is, the nursery tank is pretty small, and the babies are getting older, I need to move them to the bigger tank right away. The only thing is how do I do that without her eating or abandoning them?
Also, my mom is getting pretty mad about the whole situation, she wants me to return them but I won't- for one reason. The pet store said all mice returned will go as feeders. I can't do that to the poor babies :( So my mom will only let me have the cage I have now, 2 five gallon tanks, and the nursery tank. 1 five gallon tank for males and one for females, nursery for the ones that are pregnant and soon for the ones that are aggressive. How can I make it possible to house the males together without them killing each other? Is there a way?

Thank you very much for reading,
Erin

Answer
Dear Erin,

You have a very common problem! Pet store attendants often don't know what they are doing when it comes to mice, and they mix males and females by accident or don't separate babies on time. Sometimes it's their breeders' fault too.

The bad news is there is no way to keep males from fighting. Although litter mates who have never been separated sometimes continue to get along, often each male needs its own cage. You will find you need to find homes for the male mice.

The rest of them can stay together. It's actually not necessary to separate mice when one is pregnant. They help each other with the babies. However, 5 gallon tanks are dreadfully small even for a couple mice. Although your mom won't let you get a fourth tank, maybe she would at least let you upgrade to a more suitable size. You can put 20 mice in a 30 gallon tank. That's the size they sell for lizards. You can also get a big, multi-level wire cage. If you live in a city, you may be able to get a cage for cheap or for free on Craigslist.

If the mice are 12 days old you need to start handling them right away. They are far too old to be eaten. They are going to get very wild after they open their eyes if you haven't handled them a lot, and they will open their eyes in two days! They are going to be super jumpy within two days after they open their eyes.

Let me give you a link to a previous post of mine to someone with the exact same problem as you. It includes help sexing and  rehoming the babies:

http://en.allexperts.com/q/Mice-3824/2009/6/Unwanted-Baby-Mice.htm

If you have any more problems don't hesitate to ask. If I am on vacation, you can ask either of the other two. Jedediah has lots of experience even though it only shows 9 questions. Include the link to my answer in your question so she knows what information you already have.

The very best of luck,

squeaks n giggles,

Natasha