You would need to convert all of the files into the same format and/or then convert them all to DVD format.
If you could convert them all to something that iMovie will accept, you could do all your editing in there and then send it off to iDVD to be processed into DVD format.
Best formats to use, for importing into iTunes, would be MPEG4 or DV.
I use Visual Hub, no longer in production but still available, to convert movie clips to MPEG4.
.flv movies would need something different as they need to be converted from Flash to MPEG4 or DV.
I would really like to take some of those funny little .flv [or similar] that we all get in email and some utube or dailymotion etc 'movies' and put them on a dvd that can play on the home TV for the delight of all. Would the resolution be any good? It would be nice to have autoplay, a choose from the file menu and if they could be moderated to have the same basic sound output. What is the best way/program to do this on a MAC.
Perhaps a different disc for those 'feel good' powerpoints.