I'm pretty sure the titles of marshal, admiral and constable all give +2 command. Admiral gives +1 loyalty, marshal and constable give +2. I don't -think- this varies any between factions, though it definitely could, as the bonuses for titles are definied individually in the startpos files.

You probably would want your inns filled, yes. But if it's a choice between a slightly less filled in and making a 6 star general into an 8 star one, it's not that hard a choice - especially considering that just building bigger castles drives the mercs away anyway. With something like the constables palace...well, if you were a merc looking for work killing folks, would you want to hang around the cop station?