If you have multiple versions of the game, installed in different folders, then yes, you can remove the ones you are not using and it will not impact the version you are using.
Personally, I would backup the folder of the version I was using, before removing the others.
You can reduce the footprint of the game by removing films, skins, custom sounds, and/or terrains. However, if the game needs a terrain it will install it again. If you have the auto-download flag checked for skins, they will also be installed again.