But why purchase a 5-year old dog? Doesn't it make more sense to get a puppy or even a 1-year old dog from a breeder?
I think you can lump in any large breed that tends to be dominant, like rotties, pits, American Bulldogs, etc. They need to be put in their place and trained right, or they are a bad accident waiting to happen.
As for large dogs needing a more firm hand, etc. That's really a misconception. Small dogs are far more misbehaved than large dogs. The reason is because people who own a large dog recognize that an out-of-control large dog can do a lot of damage, so they tend to correct bad behaviors at a young age (such as jumping up on people, barking, snarling, growling, and even biting). People who own small dogs think it is cute when a tiny dog growls or snarls at anyone or anything, so they laugh and inadvertently reward the dog by using a tone of voice that implies approval, and saying things like “It's ok Fido” or “Aren't you a tough doggy!” Then they laugh, giggle, pet and cuddle the dog, hug the dog, etc. All these things tell the dog it is doing a good thing and the owner approves of that behavior.
What they should be saying, in the same stern voice they would use on a large dog, is “Fido No!” And then let the dog know, through leadership, that they are not to behave that way.
Just watch at a Vet's waiting room sometime, the difference in behavior of small dogs (and how their owners react to that bad behavior) vs. large dogs.
Ultimately, an out-of-control large dog can do more damage, but it is no more acceptable for a small dog to be coddled and allowed to behave poorly. And incidentally, there are more dog bites in this country from small dogs than from large dogs. It's just the amount of damage from a small dog bite is not generally as severe.