What I would like is to not be afraid that she could kill another one of my dogs
She's a Pit Bull, she can kill another one of your dogs. There's no way to sugar coat it, I'm sorry.
We have two males and a female. The boys don't mess with Ruby 99% of the time, but they have had their occasional snarks and about once a year Ruby or Riggs gets a hole or two in their face. Connor is mostly fine with other dogs, he'd prefer to hang out and play, but will throw down when necessary. Riggs has the worst form of dog aggression - all confidence based, little to no warning (to the average observer), he'll just launch and grab. We crate and rotate the boys. It's annoying, but necessary.
I guess I would like to feel that if there was a breach I could get them separated before there was serious injury.
A member here had her two dogs get into one bad snark, and one bite caused the loss of the front leg of the other dog. If it's real, it's real, and you just have to be prepared with a plan, and do your best. MOST "fights" are just noise and accidental bloodshed though.
We talked to an animal behaviorist today who felt it was sort of 50/50 that Harley could be helped. -She was discouraged by her being a pittie from a shelter but gave us a glimmer of hope since she is not dog aggressive with all dogs.
This confuses me. The only "cure" for dog aggression is to control it with training, but that doesn't mean it's gone, it means that you teach the dog to behave. We used to be able to put Connor and Riggs in downs next to each other off leash. In "obedience mode" they would ignore each other, but we never let them out in the house together.
I'm sorry if I sound negative/nasty, I don't mean it that way. But you have to understand that there's no "fix" to this. It's great that she gets along with some of your dogs, but now that she's injured one, you must accept permanent separation with at least those two, and possibly eventually more of them. However, even the most dog aggressive dogs out there often have one or two "friends."