My rescue was rehomed because in her last home, she attacked a smaller female too. The rescue that I got her from, said she would be better off in a home without small dogs. This girl had 3 homes before, I got her at the age of 2. You should not get another small dog, or female. Maybe she will be better off as an only dog, like mine. I am aware of my dogs past and do training classes and watch her like a hawk. She loves people though. May I ask why you are rehoming the older dog. Have you had her longer? I do think it is a good idea to rehome one. You should also contact a good trainer.