On the subject of ICUS, my understanding is that for a HCAP AOC, all that is needed (company mins aside) to hold a command is the relevant endorsement, IFR quals, and ATPL.
CASA changed the mins for the ATPL a few years ago, to facilitate the cadet progression for Low Cap AOC holders. Hense the 500 hour changes. The LCAP AOC requires, amongst other things, a min 500 hours Multi command under the IFR. However, and someone correct me if necessary, I think the 250 hour command option may still be available?
Either way, under ICAO annex 1 F/O's should be able to log ICUS from the RHS provided it's for the purpose of attaining a higher class of pilots licence (ATPL). For HCAP, the 500 Multi under the IFR does not apply!