If you are speaking of modern slugs even with a black powder charge behind it I would not, many of the slugs have a hollowed out conical design made to force the sides against the barrel or rifling, Most old black powder guns will not stand up to the pressure, and unless the barrel has no choke to it, a slug should not be fired from it at all. Patched round balls may be fired from muzzle loaders with no problems as long as there is no choke to the barrel.
Pellets or slugs. shotguns usually shoot shot or pellets, but you can shoot slugs. in Indiana we use a shotgun to hunt deer and use shotgun slugs, better to use a barrel with no choke in it. cylinder bore preferred.
of course you can
yes for a shotgun you need a rifle barrel DO NOT shoot slugs if you have a poly-choke!
Yes
yes
Yes, but an easier method would be to sprinkle salt onto the slugs.
Depends on how old the shotgun is.
yes
If it is not choked to tight, yes.
Needs to be examined by a gunsmith.
Your Iver Johnson will shoot rocks if you want. the biggest and single most issue you need to keep in mind is the breech pressure modern loads produce. most antique guns utilized black powder as opposed to modern smokeless powder and with that your comparing apples to watermellons. through the company "Balistic products" they specialize in shotgun paraphenaila and offer many books and products to safely relaod black powder shells fire antique shotguns.
Youve got the wrong gun If u want to shoot slugs get a air rifle or a slug gun