I don't see why the Great Being cannot be a Toa. Taking over a Toa would definitely be great for him to observe the Matoran Universe. But if he was a Toa, he'd be expected to defend Matoran, take an active role in events, and follow the Toa Code. This seems to be a bit too much for a being who simply wants to observe. Besides, remember that the Great Being put his spirit in a biomechanical body, meaning the being he is pretending to be has no destiny given to him by Mata Nui. Would he really have any Toa powers? It's like putting on a suit off armor; he shouldn't really have any powers. Then again, we don't know what the Great Beings are capable of, but still....I must say that a spirit possessing a body with no powers was seen when tren krom possessed lewa's body and he had no access to lewa's powers so that is likely. Also after reading some more into this i have one big question: What if the great being in question is how could you say evil or was shunned by the fellow great beings? If you think about it why would he have given such information to a bunch of (Predator)-like vorox about toa and how to halt their powers which if spread to other beings even the dark hunters could be fatal for the dwindling toa race. In my mind that raises a flag that he could be evil. And i really think that those vorox will someday spread that information.