Morning Juzz,
I don't know how well a stinger hook would work on a live bait, but you never actually said you were losing a live bait i think the mullet would die pretty quickly if you used a stinger live and it wouldn't really swim.
But if you're using a dead mullet sure rig it how ever the hell tou want just make sure you conceal the hook well where possible.
Your assumptions for the size restrictions are pretty much correct yes under 15cm you can only use for live bait, with a bag limit of 20, and sea or Bully mullet are over 30cm, and also a bag limit of 20.
Im not sure why the gap in between apparently at juvenille age, it is extremely dificult to tell the difference between the many different species of mullet but i think only Sea, Bully Mullet may be of only one species of Mullet, as the word Poddy really only refers to the juvenille in any species.
This would explain the gap as well i hope this helps mate, in my head it sounds good and i think i may be correct
Best Regards, Cheers and Beers!!!!
Mick