Can I ditch the Preview_PICTURE_SET entirely? and just use another instance of "PICTURE_SET" for the preview purpose?
Yes, you can. Just make sure it's not picked in the event where you assign random animations. You can use a boolean instance variable isPreview for this.
When you need to pick the preview instance, use "isPreview set" condition
When you need to pick all random instances, use "isPreview is NOT set"