You could edit a site using FP that was created by any other editor. The problem is when you save it. FP adds a bunch of extra code. If you want to see it, you can click on the HTML tab. To compare, go to another website to and view the code. You won't see all of the "Microsoft" or "MS" stuff in it.
An "include" page is the code that is "included" in the page. Generally speaking, it's a php page that is invoked using a small script...such as <?php include("includes/some_page.php");?>. This would call the file "some_page.php" and place it into your web page at that particular spot.
The only concern I could see is how well FP "plays" with php files. I don't really know. When I used FP, I was using it to learn HTML. I then got an actual HTML editor when I started learning PHP. Maybe someone with more FP experience can answer that particular question.