I'm working on an '84 SE I picked up cheap. It has a broken off mirror on the drivers side. When it was broken off a piece of the door skin went with it. A piece between the mirror and the top of the door, about 3/4 to 1" wide and 4-5' long. Can that small piece be fabbed and glued in or do I need to get another skin off any Fiero as other years won't fit '84's, and cut that piece out and glue it in? I don't have much experience with fiberglass.

These parts are not fiberglass. It would be best to find another door panel (you will need an '84). You can epoxy that part in and such, but others will have to chime in. I had this on one fiero and I just glued it back in with some epoxy and didn't even bother finishing it off. It looked ok for a daily driver

Just replace the entire door skin, why bother with trying to repair the plastic? If you can't find an 84 door skin, you could just replace the entire door with another year.

Thanks for the advice.