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Yes
Have it checked over by a gunmsith first. No reason it can't if in good mechanical condition.
A Rossi 20 gauge single shot shotgun can fire any factory-made rifled slug, as long as the barrel is marked Modified, IC, or Cylinder. If it's marked Full, it may be too tight for some slugs. Remington Slugger is a suitable brand.
Yes
With a well placed shot a 12 gauge slug can kill a black bear.
Yes, if it is a 10 gauge shotgun.
Depends on what it is choked.
You shoot at what is called a "patterning board" to determine the shot density at different distances (assuming that you are shooting shot and not slug). Your best bet is to buy a book. Check out Amazon.com or DBI books.
Yes, I have actually done this myself. I was out hunting with my cousin and I thought he was a deer. My instincts kicked in and I shot a slug straight at him. Luckily, I'm a very poor shot and I hit his model 1000 instead of him, but let me tell you, the slug went right through the gun and kept on going. So yes, it is very easy to shoot a slug through a 12 gauge model 1000 Smith and Wesson pump shotgun, as that's the exact model my cousin Darrell had been using. I don't know why you'd want to do this, but I guess you can do whatever you want with your guns.
Yes you can. Lead Shot is Lead Shot. I shoot all kinds of different sizes of shot out of my Mossberg and Remington Shotguns. What you do not want to do is shot a slug out of barrel made for shot. Get a rifled barrel for that.
no
A single, non-rifled projectile shot out of a rifled bore shotgun (specifically a slug gun).