As was previously stated, you just need to keep them separate. I'm curious to know, also, how long have you had this new girl, and how long did you wait to try and introduce her to your older dog? Contrary to popular belief, spaying/neutering doesn't change a dog as far as being hyper, and most times doesn't affect the level of aggression coursing through their system. As Ames stated, it MAY help, but it's not guaranteed. It is wise to go ahead and get her spayed anyway, though; especially if you don't have registration papers from a registry and plan on showing her. Dog Aggression can be worked and dealt with, perhaps the best way being just to keep her separate from other dogs. If you don't feel confident you can protect these two dogs from further fights, then you should seek out another option. "Pit bulls" are not meant to have dog friends, though several of us do maintain multi-dog homes and can get our dogs to behave themselves, as desired. This usually comes from being an experienced owner in this breed, and I do not recommend you trying it if you're not experienced with the breed. Best of luck to you and I hope you can get things worked out.