There are no good hollows. Nell is Yachiru, the big Arrancar is Kenpachi and the skinny one is Byakuya. Yachirus Shikai has a similar ability as the one from Aizen, so they wont get easily recognized by SS or Hueco Mundo by their physical apperance or Reiatsu. They used some kind of gigais that are currently their placeholder in the SS Divisions. Wouldnt this fit the story line? We would then have all the douchebags from SS, who became super duper best friends with Ichigo together in Hueco Mundo and the chances the Hime rescue would succeed just rised from 0 % to 1 %.