The two of you are talking about two completely different issues, though both deal with QuickTime.

For the mp3's, they are not movie files, but QT audio files. To have them play in iTunes, simply drag them to the open library window. You can easily change them to iTune files with iTunes Associator (versiontracker.com).

As for mpeg's. It is likely that what you have downloaded was once uploaded in tiny chunks to a binary usenet group and then reassembled in a way the Mac can only partially read. Your first choice here is Rosetta (also versiontracker -- shareware, but only a nag screen). Choose "Fix" from the menu, and Rosetta will disassemble the file again so that you can reassemble it again (menu choice: Join).

If Rosetta gets errors or yields strange results (4 K partial files for instance), then your only choice is to demux the mpg with BBDemux (also versiontracker, freeware) into two files: audio and video, which will be labelled m1a and m1v. To paste them back together, you need QuickTime Pro 4 AND 5/6. You can have different versions of the Pro Player on your Mac if you rename one of them before you install the other. Easiest: uninstall 5/6, install 4, rename, reinstall 5/6. Open the m1a (audio) in 4 Pro, copy. Then open the m1v in 5 and scale-paste. Save as independent file, if the vid is worth all this effort.