Shadow clones are unavailable for the sacrificial process, dude. They will simply dissapear the second they feel pain in the process. However this White Zetsus are special clones that will not dissapear no matter what damage they took, zetsus are special clones, as kisame said during his encounter with Killer Bee, Naruto, and Yamato. So basically the sacrifice needed in the ritual is a living and useable body, the one that exist and constant
As for Zetsus and their clones might or might not have a soul, I'm not sure about this one. They have their own mind, they talk very much, and they have a free will, yet they can be cultivated freely with Hashirama Living Clone.