Only the dudes currently in the Shootout are made into draws by Slim's Ability.
Any of them can be made back into Studs via future actions or effects.
Any dudes who weren't effected, entering after using Slim's Ability, would be their normal bullet type barring other modifiers from attachments or other relevant shootout plays.
A dude who leaves and comes back will remember being affected, but would be still made into a Stud by the Stakes Just Rose.
The most recent effect that changes a dude's bullet type will determine the dude's bullet type.