i'm sorry for interjecting or whatever, but i think i have a pretty good grip on what's happening on Bleach right now, so please spare me a few moments to try and clarify. ^_^
arrancar are hollows that obtained shinigami powers. try not to think about them removing their masks so that they become arrancars, and Aizen doing the job. just focus on that for now: arrancars were ORIGINALLY hollows.
about hirako though, he's a vaizard. vaizards are shinigamis who gained hollow powers. they're the exact opposite of arrancars. so that means, vaizards were ORIGINALLY shinigami.
when you think about this, it's pretty easy to think about. vaizards are human form, because they WERE human form to begin with. as for arrancars being in human form, don't think about that now. the explanation for that is somewhat of a spoiler.
try not to think about ranks either. the arrancars have ranks, but the explanation behind that is also somewhat of a spoiler. ^_^
i hope that helped somewhat.
