The operator still has to prove do CASA that it can evacuate an a/c with the lower amount of FAs doesn't it? Impulse currently cannot reduce crew a 717 as the evacuation trials were done with 4 cabin crew. Makes it nice for us when you have 50pax on longer sectors.....
CASA also set limits on board each a/c based on the exit configuration and adequate numbers of crew it believes is required. This is what I have been lead to believe.
Above 16 pax an FA is still not required where the operator can show that the FO is trained in providing the cabin safety functions of a FA. This was the case with the B1900 which had 18 or 19 seats. There might be a cap on this too (say 19 pax?).