As someone who often ends up repairing the aftermath of dog on dog attacks you took a very reasonable course of action and I?d stand by your actions in a second, of course you never do know an animals intent until often it?s too late though more seasoned dog types can read body language with 80-90% accuracy though there are always surprises of the worst type.
Frankly I think that those electric fence deals ought to not count as a fence --- they are always breaking or failing on some level plus do not protect dogs from threats coming into the yard or people or kids wondering on a yard if the dog is aggressive / protective --- plus they are often purchased by someone looking for a quick fix to a larger problem --- we have even seen people try to use these things to keep problem wolf hybrids contained --- with predictable poor results.
I would say that telling the owners is strongly advisable as they might want to flush out the dog?s face / eyes and make sure he did not abrade his cornea in the process of pawing at his face.
Breed may account for 5-10% of a dogs over all personality or predisposition, the rest is environment, if the converse was true working dog people would not spend so much time in selecting candidates for police / military service, heck if half the absolutes people talk about were true you could just go grab any lab and have a therapy dog and go grab any German Shepherd and have a patrol dog. I have seen just about every breed be profoundly aggressive or unbelievably nice, yes even aggressive golden retrievers, and frankly in the right circumstances most any dog will bite --- if they are fearful of their life / safety that IS their defense mechanism, a dog?s personality has a large role in this as well --- is he or she fearful and aggressive in response to that or is he or she dominant and challenging your authority --- the latter type of dog tends to be more predictable and generally easier for an experienced handler to deal with --- they will tell you when you are going to have a problem, the fearful dog is going to fight or flight and a chronically fearful dog needs a lot of work and is in general less predictable.
Though I?m not much of a small dog person myself (often I wonder if Chihuahua translates to biting rat) and granted certain small breeds were breed / selected for ?spunk? because they were meant to chase small game or rats and so forth, I do have to come to their defense as 70-80% of the ones I see that are problem children are that way because they have spent their entire life in someone?s lap and have never ever been introduced to any part of the big broad world because some people don?t think this is important for a small dog so you end up with this poor profoundly fearful animal that will fire at will anytime it?s not on it?s owners lap, the other variation of this that accounts for the other 20% or so is the small dog that has shown aggression / dominance and because it?s a small dog the owner thinks it?s ?cute? or funny somehow and does nothing to curb it.
OJ ? beautiful dogs there got to love the giant breeds.
But you did well