I've been working on a documentary since 2008 and 50% of the footage is shot on a Canon XL 1 while the 2011 and 2012 footage is on a Panasonic AG-HMC70P and the Cannon T2i. How should I edit this in FCP so the footage somewhat matches? This is documentary so I know much is forgiven.
How SHOULD you? Well, the best way would be to recapture the SD footage via a capture card like the AJA Kona 3G, so that you can hardware upconvert the footage. Then it would look pretty good. Barring that, you can use Compressor to upconvert only what is used in the final locked cut and set the Frame Controls to BEST. This will take a LONG time, but will look decent.