Hell is going to freeze over today. I actually agree with Labra!
First- reaction to another dog isn't protection, it's dogs being dogs. Especially when it's a 6 month old puppy. It was responding to signals being given off by the other dog and reacting in kind- aggressiveness meeting aggressiveness.
A dog that is 5-6 months old does not have the mental maturity to do ANY kind of protection. Can a puppy be training in protection? Sure. We have several pups and young dogs at the protection clubs I belong to. They are trained in prey drive where there is no pressure on the dog, it's all a game that the pup wins every time. If you put defensive pressure on a puppy like that, it will most likely shut down. I don't care what breed it is. True defensive drive does not fully kick in on a dog until it is mature. Some dogs that can be 3 years. It's usually at least 18 months and more commonly 2 years or so. A dog without fully developed defensive drive can't protect.
Likewise, a Boston Terrier doing bite work is being worked in prey. He's given a win/win situation. Most of Schutzhund is prey based. Even the "courage test" isn't that much defense on the dogs part. There are a ton of Schutzhund trained dogs who won't do real life protection.
With a couple exceptions on some of the dogs here, I bet anyone who can read a dog's body language could run off any dog in this topic who's said to be protective. It might get physical- once you start pounding on a dog, few will stick around, but that's what happens in a real fight. A dog that shows signs of fear- baring of teeth, growling, hackles up- can be intimidating to someone who doesn't know how to read that language. That is NOT protection! A dog acting that way can be made to run and it'll shut down. You can permanently ruin a dog by forcing it to run off the field like that. Once it's been beaten in a fight, few dogs will want to fight again.