Time machine isn't a storage utility. It is a back up utility. How it works is it takes a snapshot of everything stored on your mac when it performs a backup. Subsequent back ups it only copys things that change. I creates "versions" of your backups that you can view as a timeline and see your computer in the state it was on the version of back up you're viewing.
If you want to use time machine to back up your stuff I'd use a seperate hard drive for your time machine hard drive. It is a good rule of thumb to have a hard drive at least twice as the hard drive being backed up.
You should be able to just store your photos and videos on your my passport and use it as extra storage. I don't know what exactly you're wanting to do with them but they should be fine just "existing" on your external instead of your local hard drive.