I know nothing about Steam, but am thinking about installing FFVTTR - does it install local files that would allow me to install custom skins & meshes? FFX?
Yes it does allow all sorts of modding.
Personally, I like the GOG version, but Ive bever had problems with the Steam version.
Any full Total Conversion mods, like the ones made by Benton Grey and Prev, will be installed in separate folders and wont mess with the base game.