(1) You definitely do not need to already be Gold to do a Plat challenge. You can do a Plat Challenge even if you've never flown AA in your life. Keep in mind that if you do a Plat Challenge, you do NOT receive Gold status or associated perks upon crossing the 5,000 Q-point mark. You hold vermin status all the way up to the 10,000 Q-point mark, at which point you earn the Plat bennies.

(Therefore, for most people who have domestic AA activity - including a mix of short- and medium-length segments - the Gold Challenge followed by Plat Challenge is the least-stressful way to go, because decent benefits arrive at the 5,000-point mark.)

(2) The Challenges are driven solely by Q-points, not miles. Therefore, if Airline X posts its mileage in your AAdvantage account as "qualifying miles" (and 0.5x Q-points, for discounted economy), you can use its flights for your Challenge. (My girlfriend completed a Gold Challenge with one $350 round-trip ticket to Europe.)