The 500 hrs multi command before gaining command on a regional is only required in the absence of an approved training structure. If you have a High-Cap AOC you have that approved structure (Sunnies, Eastern, NJS, Airnorth when they get the 170 running). Without that AOC you may still have the approved training structure, and I suspect Rex would.
The 500 multi command is really only a legal issue for the likes of a small Metro operator or similar. While jet operators may state that as their requirement, that's their decision, not a CASA requirement, as evidenced by Virgin Blue dropping 500 multi command in favour of 500 multi.