I really dislike the term "rifled slug" It's very misleading. Those grooves on the slug are not rifling, they do not serve the purpose that rifling in firearms generally serves, which is to spin the bullet.
Those fins are simply there to allow the slug to squeeze down to a smaller caliber when firing through a choke without creating unsafe pressures. They do not impart any spin on the slug. Rifled slugs were created back during the depression, the fins were to let people shoot slugs out of their regular bird gun (since back then you could hardly afford to buy a dedicated slug gun). Those old shotguns typically had fixed chokes that you couldn't remove for shooting slugs. So the fins were added to let the slug pass through a choke.
Essentially you use "rifled" slugs in a smooth bore barrel and sabot slugs in rifled barrels.
Using rifled slugs in rifled barrels is possible, but the fins create too little contact area between the slug and barrel for the barrel's rifling to have any real effect. And generally causes leading in the barrel.
Using sabot slugs in a smooth bore is just a waste. They're expensive and without a rifled barrel to give them a spin, you may as well be shooting the much cheaper lead slugs.