Idealists, such as GC or Blood Dragons, may get on well with governments which are hostile to their local government.
It all depends on the circumstance, really... it's not as simple as 'You are unlawful, I am lawful, I must shoot you', I'm afraid.
Pirates, not so much. Idealists, yes. On my Bundschuh, Hans Krieger convinced me to go after Harvester_11. Someone asked why I was even talking to him as opposed to just PvPing him, to which I could safely respond "Because I respect the RM" (in-RP as well as ooRP). If there was someone attacking Rheinland civilians (Republican, Daumann, Kruger pilots/executives count as enemy combatants for Bundschuh, btw), I'd fight alongside RM in a heartbeat.
It's workable, but shouldn't be overdone. The GC landing on Bretonian planets is a little fishy, though- Bretonia wants Liberty to come into the war on their side, and they really don't want any media scandals to hit the news. Off the record and off the planets, however, I can see them helping each other beat up KNF/Samura/Kishiro, and just flying past each other elsewhere.