Sorry but Reaper King is not a valid commander. Because it allows you to play it for colorless it is fits the criteria to be a universal commander so It is banned. It is find for EDH other than the fact its not a dragon. However you should look at alternatives other than the reaper for use in both styles.
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There is too much debate and you are looking over the obvious. Planeswalkers are "Players". Effects that target player effect planewalkers not abilities that target creatures.