Bushy,
You do not need an ATPL to hold a command endorsement on a multi crew above 5700 kg aircraft.
You do however require an ATPL to be the PIC of said aircraft in Charter or RPT.
So a CPL holding Cadet can not ever be PIC of a Metro23 in RPT or Charter. Purely and simply because the aircraft is above 5700kg, although can do in a Conquest.
This little gem finds it way into CAO 40.1.0:
10.5 The holder of a commercial pilot (aeroplane) licence may log as time in command the total time elapsed during his or her command, in flight, of an aeroplane. He or she may log as co-pilot the total time during which he or she serves as co-pilot.
Command does not mean "at the controls", it assumes you are qualified in all respects to be the PIC, such as an FO on a B747, whereas the SO probably would not be qualified.