As Zippy already said the spiral cut "rifling" in a slug allows it to swage down and fit through the constriction of a choke. Some information out there says it's safe to fire slugs through any choke up to a Full. I have no data to dispute that. I have fired .410 slugs through a fixed full .410 single barrel before. The results left much to be desired, but it didn't damage the gun. I don't think it's something I'd do on a regular basis though.
Cyl, Skeet, IC, up to Mod should be good. Depending on slug brand and barrel you may get better results toward the tighter end of that spectrum or more toward the more open. Most purpose built smoothbore slug barrels are IC choked FWIW.
Again, and as Zippy said early on, DO NOT fire any shotgun shell through your shotgun with your screw in choke removed. That will damage the very fine threads that hold your removable chokes in place.