You don't need to make it too complicated Frois, but it should be solid. Ranks that have simply to do with who is the greatest and who isn't will make it so you're constantly promoting people from one to another. Ranks with well defined purposes (levels of leadership, career, etc.) will allow you to create a fun interacting structure, which is actually easier to manage (because you have less temporary ranks.)
I'd also change the name. "The Assassins" is poor... Call it something along the lines of Dakkru's Gatekeepers, Silent Hand, or whatever.
Another note: I consider "roleplayed" fights are crap when the outcome is decided simply by one fool's gut. The animated shadows start coming in, along with the blazing eyes of doom and stuff like that. Geez, it's just so messy. Also, they totally disregard the work our developers have with the mechanics and they usually involve someone sighing with deception. However, I totally agree that if in a situation you've gotten behind someone in a dark alley/room and that character is (genuinely) unaware and you've got a dagger near its neck... Then it sure counts as a kill... If the target player agrees.
Still, your members should have some skill with daggers (you can make sure they do by seeing which types of monsters they can kill with it) and some agility as well (are there items that require you to have some amount of agility?)