The best thing you can do for your tiny dog is to treat them like they’re a Rottweiler. Whatever behavior they practice, if it wouldn’t be ok for a Rottweiler, it shouldn’t be ok for your tiny dog.
Most of tiny dog aggression is perpetuated by picking them up. They can act as big and bad as they want because they know eventually you are going to pick them up and save them from any situation. That is why little dogs often instigate fights with big giant dogs. They know that they can give 110% because they will not have to back themselves up.
So don’t let your dog sit on your lap and growl at people, and don’t pick your dog up right after they’ve snapped at someone. Treat them like a dog!