Based on what you said, it seems that Buck is only protective when it comes to you guys (I am sure including your cat). Since he is now part of your pack and so is he to yours, then his protectiveness is understandable. However, since he feels that there is a need to growl at other dogs (specifically, those not belonging in his new pack), then it means that he might be taking a more dominant role.
What he did to your cat is a perfect example of his leadership. He growled at her and chased her away but not in a sense as to attack her.
So, what's there to do? Re-establish your role as the pack leader. If that requires for you to correct his behavior when he growls at other dog, then that's how it should be. Try to take him out for a walk around the block, but be careful to avoid heavily dog-populated areas. The trick is to introducing him one dog at a time, showing him that growling at others (or baring teeth) is unacceptable.
By the sound of it, I am sure he'll pick it up easily. Good luck.
