RAS can be initiated by either a Supply ship or a Destroyer, correct?Short answer: 'Yes'.
The five player variant note regarding the Destroyer player needing to leave 4 movement made me think of this.
In the main rules, it seems that only the Destroyers can initiate a RAS.
It is perhaps more usual for a destroyer to move to the Supply ship so that the former can complete all it wants to do during a turn before the RAS move ends the available actions but, as for the rule allowing the Supply ship to move to a destroyer stranded without fuel, the Supply ship may be moved to the destroyer. The requirement for the destroyer(s) to have 4 movement spaces remaining at the start of a RAS move applies to all player variants (page 12: The four squares moved while conducting a RAS count towards the total movement the ships are allowed in a turn ... ).
[The five-player variant introduces a structured order of play to combine the Soviet roles].