Interesting Alt Costume Engine limits:
They cannot change the characters voice.
As a rule, the costume they are choosing have to be the same character. So no Rosalina, Daisy, Shadow, or genderbending. I have no idea what this means for clones like Solid and Naked Snake or evolutions like Pichu to Pikachu/Igglybuff to Jigglypuff to Wigglytuff or alternate world counterparts like Mr. L or reincarnations like Link and Zelda and Ganon though I am guessing incarnations in LoZ are technically the same person so it is a possibility.
As we suspected, it's basically an advanced form of cBliss.
They might have gotten the voice clips here