Thank you everybody for all your help and encouragement. I really appreciate it.
Unfortunately, we had somewhat of a setback last night. I had been feeding the dogs in separate areas, but I forgot to put Charlie's food up and he was laying near it when Sara approached. They got in a pretty nasty fight. We broke it up and both dogs seem to be okay physically, however Sara is now afraid to leave her crate. I'm guessing this will just take time? I was also thinking of getting them in the same room (but leashed) and rewarding them for looking at each other, although I think this is only an issue with resource guarding. Charlie doesn't seem to hold a grudge and will try to kiss Sara, but she's not having it.