My bc, Eli is exactly like that. But, mine has both sides. He is sometimes extremly sensitive to a certain type of dogs. But, at the same time, he is extremly genourous on other types.
My guess is partly that when he feels insecure (a bit afraid), he barks and lunges toward other dogs, which usually are bigger than him - shephards, labs, etc. (he has been lucky not to be engaged in a big fight).
At the same time, when he meets some other dogs which Eli wants to play (I know this from Eli bowing position - he lies down as a sign of invitation), he gets also frustrated because I block him out of fear that he might lunge toward the dog. These two eventually intermingle together so that I have no idea of what would be going on whenever we encounter other dogs.
I know this Eli's positive side because he gets along with cats and other small dogs in a closed, secure environment. When I ask my sister to take care of Eli (because I have to leave town some days), he has to live with a cat and four small shitthus. He never growls or be agressive to them.
So, my guess is that if op follows his gut feeling that giving some opportunites Salty to meet other dogs in very secure environments, Salty may get over the uncertenty with other dogs.
In the below, Eli barks at this cat, but, it's very friendly one. He still barks and run for cats and birds in the streets.
