All Fiero body panels from all years are interchangeable. So you can get parts off of an 85 GT or 86/87 SE. Yes you remove the small rocker and put on the lower skirt. You then need to get the door, fender, and quarter pieces. To install them you need to remove the panels and drill some mounting holes. Then the front and rear bumpers.
If you want the later GT fastback body you will need to get the entire back clip and the front bumper. Also the black trim for the doors/fenders and side marker lights as this uses the smooth trim not the ribbed trim.
All the body panels bolt on.
Also just so you know, If you have a solid roof and you want a factory sunroof you just need the roof panel, sunroof hardware and glass, and the headliner. swap them out and you have a factory sunroof. The hole in the frame is already there so it's just a matter of unbolting and bolting on the new panel.