For iPod, you need specific size & there are audio requirements too. Check How to transfer to my iPod (& iTunes) - for video as it has a link to iPod requirements.

The file format needs to be either mpeg 4 or H.264. So mp4 is a good choice. Hover your cursor over the file in the search results to see if it indicates any viewing dimensions. eg: 320 x 240 or 640 x 480, etc.

Other than that, you can convert video files to iPod compatible format, but that will mean some loss of quality. And some converters are not easy to use.

From what I see, mp4 audio (or m4a) / aac are the audio requirements, unless the audio is in quicktime mov format.