Yes you can fire slugs in a double shotgun. But I'd be careful about what slug load I used. Most modern slugs are undersized to accomodate the choke, but I'd want to be sure I had an undersized one.

A shotgun should be regulated for 40 yards, but that's for shot, not slugs. Low priced double guns are likely to have had the barrels put together in a jig and not really regulated. I have always heard that the typical SxS will cross fire with slugs. Every gun is a rule unto itself, so I'd take it to the target range and put a few slugs through it to see what it does. Have it well sorted out before you go out to deal with a hog.

The Paradox type guns were actually more rifle than gun and were regulated for either a round ball of a bullet. There is another type of gun, the ball gun, which has a true cylinder barrel and is meant to shoot a round ball and not shot. It will have sights and will typicaly be a double. But not common, usually an expensive collector's piece. Bernardelli is said to have made a double barreled slug gun in recent times.

I've no experience hunting hogs, but buckshot does not sound to me like the way to go. It's marginal on deer and they are less likely to argue the point.