Is it possible to...

Move single folder, create symbolic link.

cmd as admin -> mklink /d "C:\program files (x86)\steam\steamapps\common\skyrim" "D:\new folder\whatever path\skyrim"

With this. Make sure you run as administrator.

Or get an external HD and every time you download a game from wherever you can store it there to save you the DL time or just move games around that you stopped playing etc. That's what I did and it's much faster than having to reDL a game.

Yes, with symlinks. They basically better shortcuts. You can create them via command line but if i'm not mistaken there are already programs with a gui that are designed to especially move Steam games

edit: steammover for exmaple but i didn't try it so use it at your own risk

Symlinks are indeed the way to go here. If you want to have the complete control over symlinks (and a bit more) with a very neat context menu integration into windows explorer, I recommend Link Shell Extension