We recommend submitting clips in the highest quality format you have available. The best format to submit is MPEG2 (dvd quality) files - we will automatically derive your MPEG2 into smaller video files. Feel free to upload the same clip in different formats, if you'd like. Let us know in the forum at the bottom of the page when you've uploaded a stock footage clip to open source movies and we'll move it over if it fits the requirements.
There are many other videos in the archive that you can use in your own movies. For example, you can search the archive for all public domain movies, which includes all of the Prelinger Archive and many more high quality, interesting films. If you'd like to learn how to search the archive by CC License-type, please see this search FAQ.