I agree. Tomorrow is our last session with a behaviorist for dog aggression issues. All sessions were done in house and we used our everyday collars. Things aren't entirely perfect but you can't expect miracles in a few months, but the situation is manageable and the more we work at it the better it gets.An e-collar is not the answer to the problem. If anything, it'll make it worse. Sorry that's not what you want to hear.
Contact a behaviorist that uses positive methods. Until then, keep the two dogs separated.
It also sounds like the GSD has become a helper dog/bully. I've worked with numerous dogs that feel the need to help supervise the other dogs.
Forget the alpha mumbo jumbo. You've been given solid advice on the matter. The dog shouldn't even know that you're delivering the shock from a collar let alone viewing the remote holder as "alpha".
I will add, the "pack leader" mentality was not used, but the "I am in charge, not you, I will control the situation and dogs" was. Basically my DA dog had to learn that I was going to take charge and she didn't need to worry about it.