The last ability of Slime Against Humanity lets you ignore the "four-of" rule. It doesn't let you ignore format legality. For example, during a Murders at Karlov Manor Limited event, you can't add Slime Against Humanity from your personal collection.
If you own face-down cards in exile, they won't count toward the value of X, even if you're allowed to look at them and you know they are Ooze cards or cards named Slime Against Humanity.
The last ability of Slime Against Humanity also lets you ignore the "singleton" rule in formats like Commander.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Jon Vermilyea
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Slime Against Humanity isn't put into your graveyard until after it finishes resolving, so it doesn't count itself for its own effect.
