Currently as the game stands, legacy names and surnames are the exact same thing. They are also unique across the whole server. This means no one can be related except your own characters. Which means people trying to play together as a related pair, are SOL.
2, There is the legacy system whereby you choose a last name, not sure if it has to be unique to your account though
Sorry, this is how Bioware has deemed it, and they are wrong in doing so. We can only hope they realize their grand mistake, and detatch surnames from the legacy naming system... and do it better.