This is just my 2 cents but having had lots of dogs come and go through rescue I find the boys adjust a lot easier than the girls but given time and patience the girls have always worked it out. The girls have always been a little "bitcher" to each other and also to the boys
but they do work it out. Mine are all spayed/neutered so I am sure that has helped somewhat.
I am guessing this little girl is under a year of age so still has lots of puppy in her but probably has learned to get along with older females.
She sounds adorable. Ask the breeder if you can have a trail run with her to see how everyone gets along. It will take a couple of weeks to see if they will accept her.