In my most humble of opinions, it is safer to keep mice in a tank or barred cage. Those little buggers are VERY good at escaping and can chew through a number of things. Also, they are excellent climbers and jumpers. Definitely don't worry about keeping them all together. As long as you keep a two or three female to one male ratio, you are fine. If you notice any of them being aggressive, you can always quarantine and introduce them to their own colony later.
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