Is it true that all dogs get along when they are allowed to run free?

No this is a persistent myth. It is true that most dogs meet without difficulties when they get to know each other without the restrictions of the leash. Off the leash they have a chance to observe all signs of dog communication. But dogs, even off the leash, can have mis-communication. They can fail to get along and it is not guaranteed that they will not, more-so if not properly socialized, get along.