why what happened, why is it messed up?
I breed mice and when old mice die i have to introduce new females to colonies, now if anything i should have the biggest problems as my females are pregnant and lactating, and therefor full of hormones and extremely territorial, however there has only ever been one incident where i could not allow a young female to stay with the colony because of a certain very aggressive female, however when she passed away i was able to put new females in the colony with no problem whatsoever, even without cleaning the tank out!
So yours shouldn't be too hard considering they aren't pregnant or breeding!
What my suggestion is, no need to clean the tank out, take your original females out of their home and locate them somewhere they will be safe for an hour, put the new females into the orig tank and leave them to explore for an hour or so, in this time they will smell like your orig girls and also be spreading their own scent.
Then you reintroduce your girls, make sure theres food and treats to keep them occupied, maybe some new toys, i put new loo roll tubs and big piles of tissue paper in the tanks, keeps them occupied for hours
Just observe, the females may chase and appear to nip at the younger ones, and its possible the younger ones will scream their lungs out, but they only do it because they're nervous not because they are hurt to don't worry too much. My advice is if theres no blood then don't interfere.
Put another hide in so that they are not forced to use the same cover. and tbh the best thing i've found is to watch for half hour or so, if nothing terrible has happened then walk away and leave them for as long as possible, or else you just end up watching and worrying.
I do this all the times, and i've never had to take a mouse back out however much they are squabbling, i usually leave them overnight and by the next morning they are in the same hide
males sometimes take longer, i remember my females wouldn't accept them for about 3 days, but all is always well in the end!
Female mice tend to be all talk, theres lots of sqeaking and rolling around but they rarely do a lot of damage to oneanother.
and with your young ones being young, you've got the best chance now at introducing, its much harder with adult specimens!