Is it safe to shoot slugs out of a 20 gauge single shot shotgun with a fixed modified choke?

Yes, it is, but make sure you shoot foster style slugs. They are easier to force (swage) through the choke, and are designed for smoothbore shotguns. I like the Remington managed recoil slugs. A 1 oz. slug coming out at around 1250 fps is nothing to sneeze at, and they're very comfortable to shoot. If bears are your concern, however, I might go with something with a higher velocity.