so far it seems the levels are :
[ run of the mill hollows ]
- gillian (aka menos grande)
arrankarus are hollows which have taken off their masks... i'm guessing that the current 22 arrankarus are random types of hollows; thus the sizable difference between indivial arrankaru's powers.
i'm also guessing that Yamii probably came from a ajuukaru whereas Ulquiorra came from a vastrode. maybe D-roy came from a run-of-the-mill hollow ?
hmm... what would Grand Fisher be considered as ? considering his intelligence and battle abilities, he *might* have been an ajuukaru.