I doubt there would be any need for the Patrician to actually sic the Assassins and Thieves Guilds onto the Black Org.

You're right of course, but I tried to take Teh Ebilness of the Black Ops into account. They wouldn't go down without a fight, and they'd certainly cause some casualities... sooner or later, some of them guild bosses would turn up before Vetinari to complain. ;)

And let us not forget the trolls

Hahaha. xD Chrysoprase VS Tequila - any bets? :D