Don't the Planeswalkers in Commander 2014 that say they can be your Commander contradict the fact the Commanders have to be legendary creatures?
The very first of Magic's Golden Rules:


  101.1. Whenever a card’s text directly contradicts these rules, the card takes precedence. The card overrides only the rule that applies to that specific situation. The only exception is that a player can concede the game at any time (see rule 104.3a).


So yes, they contradict, and it's fine. And this isn't really unusual; tons and tons of cards have text that effectively says "now the rules work a little differently, and here's how." For an extreme example, the rules say you lose the game when you're reduced to 0 or less life, but Platinum Angel says you can't lose the game, and it takes precedence.