actually he did witness his power, or at least learned from someone what it was, and he went to look for the flour to use it against him... he simply told Luffy to use it in the fight......


as for Nico Robin... it was a stretch of her powers... i dont remember in the manga anything saying what is her limit, so they had the door wide open for possibilities, but not even Luffy can stretch his arms as far as he wants... so there are certainly limits...