Well, in my humble RP opinion I don't think you should limit yourself with only the ships that Xeno NPCs fly. But should you fly any ship? No hehehe.
Something tells me that piloting an Order fighter sounds better than a Corsair one, I would also as you well pointed not use any lawful faction's ships. My hint, stick to the civilian/border world which are "neutral" ones. Even if other Xeno NPCs don't fly them, I don't think it will be out-of-rp.
But even an Order fighter sounding better than a Corsair one... well, I would not use any other faction ship. So, UNDER MY RP OPINION:
GREEN (very tolareted by me): Any civilian, border world or any other kind of "neutral" ship.
YELLOW (not much under my RP opinion but I will not complain that you are out-of-rp): Another pirate/terrorist faction ship that has no war with Xenos.
RED (out-of-rp in my opinion): Any lawful faction or a declared xeno enemy (rogues, etc.) pirate/terrorist faction.
