When using Hoopa GX's "Devilish Hands GX" attack, can I keep choosing a Pokemon that would already be Knocked Out by previous choices, or do I have to choose a different Pokemon once I have done enough damage to KO the first one?

You make all 6 choices before doing any damage; so if you wish, you can do more damage than you need to a Pokemon in order to knock it out.

Relevant cards

SM9_166

SM9_187

SM9_96