As you've been reading, yes you should only shoot sabot slugs through a rifled barrel.
1. Shooting foster slugs through a rifled barrel can cause a leading effect, gumming up your barrel with lead, leading to a few problems.
2. A rifled barrel is meant for sabot slugs which will broaden your options to the higher performance slugs which excel tremendously over foster slugs. you should test several different groups of rounds to see which one your gun will favor.
You should research the twist rate on your barrel's rifling. the slower twist rates will print better results with slower slugs (lightfields, hastings, etc..) which many people completely swear by the accuracy of these slugs. a faster twist rate (1:26-28) will accommodate the faster slugs better on paper with incredible performance on game. winchester partition golds are a great start. also the remington corelokt ultra are great too, but try a bunch..
Also, foster slugs aren't "rifled", as the name rifled slug portrays. I'm not sure if the "riflings" on the slug actually imparts any spin onto it but the main purposes of the "riflings" or grooves on the slugs are to
1. reduce contact between the slug and the barrel --> reduce friction.
2. allow the slug to be compressed more easily if encountering a choke.