I was always told that foster slugs will lead the bore of a rifled barrel and you shouldn't use them
Lead slugs may lead a barrel and they might not. With sabot slugs you might find plastic fouling and you might not. It's all part-and-parcel of the game. Any time you force something down a tube at high pressure, you're liable to have fouling. The "rifling" on a Foster slug is not really rifling, it is designed to minimize the friction in the barrel, to allow the ammo to be shot in barrels that might not be exactly bore-gage, and to allow the slug to be fired from choked barrels. The rifling fins are designed to swage through a choke.
I believe that rifled shotgun barrels are best served with sabot slugs, but there's nothing necessarily wrong with using Foster-type slugs through a rifled barrel. We've been pushing lead bullets down rifled barrels for a couple of hundred years and have learned how to deal with lead fouling.
We've also been using round balls for lots of years, probably longer than we've used rifled barrels. One of my most accurate muzzle-loading recipes uses a patched round ball down a rifled barrel. It's accounted for several deer. There's nothing wrong with that either. If your shotgun shoots best with round balls, use round balls. In honesty, though, I bet you'll find the best accuracy with a sabot slug.