I ride a 7.5 fx. My only concern of breakage would be the wheels themselves. While I take it on the occasional dirt path with no problems, the bike becomes a real handful in gravel, sand and mud due to the tires. I would not consider it an appropriate "off-road" bike. I am not sure how you would define "light" off-road. The carbon fork is also of some concern
You could always get the 7.6 and still keep your MTB for trail work. Any bike that works well off road isn't going to be great on the road,and vice versa. Use the hybrid for the street and the MTB for the trails.
If you only want/have room for/can afford one bike,then you'll have to bias your decision as to which type of riding you do most often. Kona's Dew series,C'dales Bad Boys,and Specialized's Globes are all street bikes that can handle some light trail duty. Specialized's Crosstrails and Jamis' Allegro X's can handle more serious off roading,and are ok on the street.
C'dale BBU('05 and '09)/Super Six/Hooligan8and 3,Kona Dew Deluxe,Novara Buzz/Safari,Surly Big Dummy,Marin Pt Reyes,Giant Defy 1,Schwinn DBX SuperSport/Qualifier,Brompton S6L,Dahon Speed Pro TT
I have a Trek 7300 that I have really beat up off road a couple of times. It's no mountain bike what with the thinner tires, but that cheap suspension fork and seatpost do take the shock out of any jolt. I ride that bike all over and pretty much over anything.