You don't have to have finished URU and To D'ni in order to play POTS, Teresa, but you do have to have URU loaded on your computer. There are some things that you will have to go back and do if you don't have a completed version of URU (for instance, you will have to start the power in Gahreesen so that you can access Baron's office), but that wouldn't be too difficult to do. Also, your Relto will look pretty bare, but you could run around and pick up the Relto pages from the different Ages if you wanted to.
If you just want to replay POTS, you could simply click on the little tab at the bottom of the book and remove any progress made in that part, without disturbing the rest of the game...