The problem? iTunes. You add that one video via iTunes? Now you have to wait for the six plus sync process. Horrible if you're in a rush...and if the video isn't in MP4/h.264 format that matches what works? Again, you're SOL.
There's a way to totally avoid the sync process.
Get a video player app and add your video on the App section of itunes. Today gplayer is free. Add your video to the Apps Tab of your device...go to the bottom and choose an app at the bottom that has file sharing capabilities.
Bonus - Apps like Gplayer will play video that doesn't conform to hardware playback.