I have 3 female mice housed together (at one point it was 6), and I have never had any aggression problems. They wrestle over food, however, lol, but they have never bitten each other. I would suggest keeping a really close eye on them for a while and try and see what exactly initiates the tail biting. Is it over food? Is it territorial over a certain toy or house, etc? If there's nothing you can change to make it stop I would suggest seperating them. Normally, mice are social and do not like to be alone, though, so I am very suprised. Especially considering the recent onset of this biting and their age!
Good luck, and keep us posted!
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"Heaven goes by favour. If it went by merit, you would stay out and your dog would go in."
Proud mom to Halle (dog), Sugar (cat), Tiffany (beta fish), Mango (molly fish), and Macy, Paris, Tessa, & Ruthie (mice).
RIP Kiska... the greatest...