Remember that Nihilus is still sentient, despite being driven by hunger he is not like a rancor who gobbles up everything. He orchestrated several elements of the Dark Wars including his alliance with Vaklu. So its obvious that Nihilus can be manipulative, cunning and use diplomacy. For that reason I don't think Nihilus would just turn on his allies and consume everything. He is capable of just consuming the Sith Emperor and being done - remember he once served Traya (I assume this would be their relationship) and had an alliance with Sion.
That is true, I forgot about that, also wouldnt Traya also be a wound in the force since Nihilus had once stripped her of it and it seems that she and the exile got it back in the same way through each other?