Use Generic Unit, same as Bethesda when they make their games using 3dsmax.
If you're gonna make tilesets, try to keep them 256x256, 512x512, etc. Height only matters if you plan to make them stackable. You should always try to keep your polygons count as low as possible, ie a plane surface doesn't need subdivision unless you plan on having vertex shading, or the smaller the cylinders, the less sides you need to preserves the illusion of roundness. In 3d modeling for games, less is always better. If your stuff is optimized it means everyone will be able to enjoy it.
Yes you can use vanilla textures on your new meshes, but you're gonna have to work hard in the UV mapping department to make them fit. Its not like with Oblivion textures where you had multiple textures for one mesh, they were simple and could be applied to almost anything. The Skyrim textures, like FO3, are based on unwrapped UV maps, meaning they're complex, and you often only have a few per nif.