Sullivan is a natural halfback. Modern five-eights are built quite big nowadays, and halfbacks are (generally) a lot smaller. If you watch some of the games he's played, although Sullivan does have a solid running game due to his speed (making him good with support especially like all good halves do), his kicking is mainly what stands out for him - when you're playing in the juniors, running even as a halfback is a lot easier. A good kicking game is usually what takes good young halves from reserve grade to first grade, and Sullivan has exactly that. He's not a five-eighth like Amone, because Amone is a lot larger and has a much stronger running game. That's how they won the SG Ball final.
Corey Norman is not a natural anything, especially not halfback. He used to be a solid player in his early days and then it just went downhill very quickly