I'm 100% fine with my slightly knobby 28s (Schwalbe Marathon nonslick armored commuter tires, an awesome choice) on slightly rough fire roads. You have to pick your line carefully if the road hasnt been groomed recently but it's more than doable. **** I ride my 25's on fire roads but I am a special sort of idiot.

edit: ah i see you know about marathons already, good on ya.