You're going to be grouse hunting during modern firearm deer season? If so I would carry your deer rifle, and maybe a 22 caliber handgun if you really want to shoot grouse while deer hunting. Or, if using the shotgun, hunt with it loaded with slugs for deer, and if you see a grouse you want to shoot, try to quietly and quickly unload the slug and slip in a 7 1/2. I don't know, just for me deer takes priority over grouse. You can hunt grouse up to December 31 while deer is a much shorter season.
But for a chance encounter with a bear, I think it's a good idea to keep some slugs handy while grouse hunting. Chances are a bear will be gone by the time you get your loads switched, but you never know, it might work out especially if you first spot the bear at a distance. For the type of slug I would just get the lowest prices slugs you can find, since you're not going to have a whole lot of accuracey anyway with just a one bead sight, and no rifling. The choke tubes are fine with slugs but you'd be best off with an improved cylinder choke. If you don't have that one, use modified. And be sure to verify the point of impact with the slugs. You'll want to limit your shots to about 70 yards or so.