I'll try parts of these questions.
You can generally shoot shot, slugs, and sabots out of either a smooth bore or a rifled barrel.
However-
With a rifled barrel, rifled slugs and shot don't do so well. Shot patterns usually get blown real wide with very little shot in the middle of the pattern. Slugs often go in various different directions than sabots do.
In a smooth bore slug barrel, sabots shoot real high and keyhole for my guns because there is no rifling to stablize them, and they are traveling faster with a completely different ballistic coefficient than lead slugs, which stablize by there design and construction in smoothbore barrels.
Rifled slugs usually do better in a smoothbore barrel with little (I. C.) or no choke. Sometime a particular barrel will shoot better with a little more choke. I have never personally seen a full choke barrel that handled rifled slugs very accuratly. That doesn't mean that no full choke will do well with rifled slugs, I just haven't seen one yet.
My read on the situation is to use sabots in rifled barrels and rifled slugs in smooth bore barrels. But if your shoulder will handle it, try all types and brands of slugs off the bench to see what your guns will do. Then you will be looking for a gas gun for sure.
Cat