Re: Starfleet Marine Corps
Indeed, it appears that SF Security is the de facto only law enforcement force in the entire Federation, rather than just within Starfleet...
But didn't the guy in "Search for Spock", who apprehended McCoy in a bar for trying to get passage to Genesis, identify himself as Federation Security?
Dr. Howard, Dr. Fine, Dr. Howard: For duty and humanity! --Men in Black (1934)