If you have 2 copies of “Cliff the Trap Remover” (for example) in your hand, you can Special Summon 1 of them with “Mustering of the Dark Scorpions”. You can Special Summon him even if you already have “Cliff the Trap Remover” on your side of the field from before you activated “Mustering of the Dark Scorpions”.
You don’t have to Special Summon all possible monsters with “Mustering of the Dark Scorpions”; you can keep some in your hand if you want.
If your opponent chains “Ring of Destruction” to “Mustering of the Dark Scorpions” to destroy your “Don Zaloog”, “Mustering of the Dark Scorpions” still resolves.