We have an older fiberglass shower unit and I would like to change the sliding glass door unit. Can the door unit be removed and a traditional shower curtain replace it? I understanding I would have to plug the holes from the mounting screws. If the traditional shower curtain and rod will work, what would be the best filler to use to plug the holes in the walls where the door unit was removed?

You will have to olay with paints to get it to match but you can use Bondo, just like a fiberglas car. ACE , KMart and most box stores carry quarter cans but be sure too get the cream hardner too.

I Would Use Some Pre Painted Screws That Were A Pan Head (white)
And Screw Those Into The Holes, With Some Silicone In The Holes First,
It Will Be A Cleaner Look Than Just Paint,

Or

You Can Buy Some Fiberglass Filler You Mix It Like A 2 Part Epoxy, And I Use A Razor Blade To Put It On You Can Do That Or A Putty Knife, But I Put Mulitiple Coat , Allowing Each To Dry, And Then It Is Wet Sanded, Very High Grit Sand Paper, Like What You Use For A Cars Paint,