[Quall999]

Being safe to remove certain files kinda depends on the game. I can remove the xdemos in halo, with no side effect. When I remove them in amped (or some other game. I dont remember the name) the game won't play.

The only files that can be removed and replace with dummy files, are the movie, sound, and picture files. But the dummy files have to be actual movie/sound/jpg files. So, you can replace a 3 minute wav file, with a 3 second wave file providing the name is the same.

If it is a movie file, you have to get the same resolutions. If the movie file is 640x480, then you need one that is 640x480 (rarely tou don't have to). However, you can replace it with a 1 second movie file and basically take up no room. (if it is one second, sometimes the movie will just hang untill you press a button)
I ussually just convert a blank jpeg image into a movie file. It plays for less than a second and barely takes up any room at all.

Making a blank file and renaming it just won't do it.

And no, not all movie files are .bik. It's just that they are the most common, and they are easy to replace/downsample.