I would imagine that a 50Gb creation would be plenty large enough for the PTS based on what PTS is installed on my drive. (38Gb)
That's fairly easy. No separate install. Just copy the "Launcher" folder from you SSD to your HDD (Make a folder for it , call it whatever you wish, like Zenimax, ESO PST etc, then copy the "Laucnher" folder to it). Start the launcher on your HDD (Bethesda.net_Launcher.exe) and install PTS. It will install to the same parent directory the "Launcher" folder is in.
I have the same setup and that's how I did it.