DIY floor pans.

Fiberglass is an acceptable material to use on properly prepared steel. Properly prepared is the key word. Fiberglass is polyester resin, same thing used in body filler and gorilla hair. Its not good to use in areas that flex because it can crack or chip. Seam sealer would be a better choice in those areas. Get rid of all the rust you can and rough it up with some 40 grit to give the resin something to bite to.
You have to do the best job you can do that your budget allows. A full panel will give you the cleanest results with less labor in the end. However, you can do an acceptable job patching if you take your time and blend everything in. It all boils down to time vs money. Good luck with your repair. I will take a look in my parts and see if I have any patch panels I can donate. I dont think I do but I will check my stash and see. Most of the cars I have parted were rusted beyond recognition so not many body panels on hand.