Quick idea. What if the total number of men you could command effectively was spread out over any NPC ships you hire?
It becomes viable for a say, Rank 4 at 120 men, sailing a trader's snow to hire a privateer at 60 men to escort, or for someone commanding 650 men, to crew a constitution and have 200 men to command a NPC cerberus.
In PVP, it is nearly always better to crew the best ship possible since NPCs aren't amazing, so most people wouldn't be bringing NPC ships if it means they need to sail a class lower ship to spare the men available.
This gives the Belle Poule and Frigate a class by themselves, since someone at 350 men can spare 70 to crew say, two cutters or something. This also lets traders sail with larger escorts since trader ships generally have fewer men required.