Joining video clips can only be done if the specs of the videos match.
In the case of MPEG-2 video, the specs must be "close enough" to coexist within the flagged MPEG container.
So ... run a Gspot analysis for me. See
http://www.digitalfaq.com/forum/vide...-analysis.html
From that, I'll know if it's possible to merge as-is, without a re-encode.
My gut feeling says you'll have to re-encode at least one clip. Given that retail source is better for encoding abuse, it would likely be the clip that needs a re-encode. Then it would match the homemade piece, and you could merge in a good program like
Womble MPEG Video Wizard.