Hullo, some advice. I keep pet rats and I've seen something like this situation before (not so long before, actually). I've got these two females that came from the same breeder and are being kept as pets, and throughout their stay with me they've been introduced to several batches of new rats that eventually ended up as snake food. One of them's particularly aggressive with new rats-- probably why I named her Mistress-- she even attacks males larger than she is.
Once, I had to house the old females with two new weaned (like 6-week old) rats (one male, one female) and Mistress immediately took to attacking them, after which I had to consult almighty Google for introduction techniques and introduction-related aggression. Found a study describing how established colonies don't take too well to new rats (new rats were killed within a few days), but I figured I had to put my foot down.
Bathed the old rats to remove scents, then completely cleaned out their cage with soap and water and put the new rats in first, followed by the docile old rat. Old rat did the basic pinning and grooming to establish dominance, and while that was going on I put Mistress in. Under close observation, Mistress chased them around a bit, boxed a little, but in the end there were no horrible scuffles and the rats got along well until the male went to Feeder Town. Still have the little lady though, and she's bonded so well with the old pair.
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