I am working with DVX 100a 24P footage (not advanced) recorded with a letterbox matte on it and canon xha1 24F HD footage recorded without a letterbox matte on it. I'm using FC version 6.0.6. The canon footage has a 16:9 aspect ratio and the DVX 100 has 4:3 aspect ratio.
Could someone please advise me on how to match the 2 footages so they both have a letterbox look?
Work in a 4:3 timeline. When you drop the 16:9 footage from your Canon into that timeline, it should shrink look like letterbox. If it doesn't shrink automatically, you may have to change the scale to 75% and/or change the Distortion parameter in the Motion Tab to -33.33 or 33.33 (can't remember which one off the top of my head).
I would recommend going for a wide screen look over a letterbox look.
Wide screen TVs are far more common these days then 4:3 TVs are. With that in mind if you play a video that is permanently letterbox on a wide screen TV you end up with a small image that has black on all 4 sides. And to me that just looks bad.