That's not necessarily true. Clean the cage with the does in it, put fresh bedding and lots of tubes and things in there (toilet paper tubes) to keep them busy. Also put vanilla on everyone's butts and noses. This way everyone smells the same, and nobody has any territory to be mad about.
If your hopper is too young, it's going to die anyways, because it won't be properly weaned. But if you go through that, you should be able to introduce them with no issues. Just give them a watch, make sure nobody's biting. They'll probably all chase a bit, but it should work out fine.
There's never been a situation where I've needed to go that far (usually cleaning the cage is enough) to introduce female mice, or even male mice to female mice or female mice to male mice. The issue is with male mice and other male mice.
There shouldn't be any real issues unless your hopper is too young, and dies.
I've never had a doe that I couldn't put in with other mice I wanted it to go in with. Not once has it ever been such an issue that I had to separate them.
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