I'll chime in with my experience, but as you'll see I'm no expert. Learn from my mistakes. I adopted what NorCal rescues would call a Level 4 dog--high prey drive with dog aggression-- when she was close to 3 years old. That was 2 years ago. She's fantastic except for the dog aggression.
1. As I believe others have said, don't push her to meet and socialize with other dogs right away. I did with my girl, and I had some big problems. Let her settle in and feel comfortable in her new home/surroundings first. Part of what's going on is her world is turned upside down so she feels everything can be a threat. Also, she probably doesn't trust you to help protect her yet. It takes time-- months as others said.
1a. During the time of her settling in, really work on obedience training. It sounds like you have a great start with the recall. If she was trained for K9 work the foundation is probably there. This will help in a lot of situations, but I would never ever pretend my dog is going to recall in every situation. Get her focus and attention on you, so if another dog comes along you can work on getting her to ignore it if possible.
2. Just write off the idea of her playing happily with any random dog or dogs that come along, or in a dog park. You just have to let go of that with this kind of dog.
3. Introduce her to only 1 other dog very gradually in a VERY controlled environment. Trust the other handler 100% to know what can happen and how to react (see Leerburg). With my girl, proximity was 90% of it. The other 10% was how the other dog acted. You don't want to try this with another dog aggressive dog of course, and in my experience, a neutral GSD/Malinois/working dog of similar size and opposite sex is easiest.
4. Hopefully, the other dog can become a play buddy so she can learn that other dogs are OK, not a threat, and even fun. In my case, I foster other GSDs since I've never found a suitable play buddy. My dog's aggression ramped way down after just our first foster. Also, walking with a 2nd dog makes my girl really comfortable-- she can meet off-leash dogs when there's a foster with her. It's amazing.
5. Never, ever, ever think that she can't snap back into being a monster over nothing. I thought my girl was doing so well, and then one day, boom, $600 vet bill on a Jack Russell. You have to be eternally vigilant for the rest of her life, for her sake, yours, and other animals around you. Again, the best thing is to find a play/companion of her size and energy, and never ever let her go in any kind of unknown situation, especially around any animal smaller than her.
In my experience, things can improve, but I'm never going to a dog park. I can now walk my dog down at the beach where everybody lets their dogs loose. But I have her leashed, and we head straight across the river to play by ourselves (or with a foster) away from the maddening crowd.
Good luck!