Ack, not all of my random options of sprites have skins. But accessing the skinCount helped me set the inner-random-random.
CCRect r = boundingBox();
_spine = new CCSkeletonAnimation((shapename+".json").c_str(), "creatures.atlas");
int count = _spine->skeleton->data->skinCount;
if (_spine->skeleton->data->skinCount) {
Skeleton_setSkin(_spine->skeleton, _spine->skeleton->data->skins[rand()%count]);
_spine->setSlotsToSetupPose();
}
_spine->timeScale = 1.f;
_spine->setPosition( ccp(r.size.width * 0.5f, r.size.height * 0.5f) );
_spine->setAnimation("idle", true);
addChild(_spine);