Anyone that repairs metal with fiberglass should have their car confiscated. Sorry, that's the damn truth. It will make the rust WORSE. It will speed up the decay of the remaining metal. You're also not going to be able to combine the two in a way that will maintain the structural capabilities of the unibody chassis, I don't care how clean you make it. The fact that you go out of your way to say it's a temporary fix and not to take passengers indicates you already know you're giving terrible advice that will compromise the integrity and safety of the car.
If you find yourself seriously considering repairing metal with fiberglass, it means you're in over your head and do not have the skills to make the needed repairs. Walk away and take it to someone that can do it right before you screw it up worse. Hell, the fiberglass isn't really even cheaper than plain old sheet metal, so all it really shows is that you lack the skills to carry out the repair. It's just a waste of time, money, and effort. In a very short period of time you'll have to do it all again, and if you didn't have the money to fix it right the first time how are you going to have the money to do it twice? Especially when the second repair will be more extensive than the first because the first repair did even more damage than was already there?
Yeah, the best way to do it is to buy a full pan. Not only that, it's probably the easiest way to do it too. It definitely takes less time and welding than if you start making small patches and welding them in. Yes, plain old sheet metal is cheaper in up front costs, but don't forget that you're going to need a lot more welding rod/wire and gas to make the repair happen. The spot welds to replace the full panel take very little filler material and a ton less weld time. Considering the entire repair, if you have to buy and extra tank of gas and another spool of wire how much did you really save vs a full panel? What's your time worth?
If the full pan replacement is off the table then j par lays it out well. You want to make the patches as large as you can without making the bending of the panel too complicated for a single patch. The smaller you make the repair sections the more weld you're going to have to lay out. The more weld length you have to share with old metal the harder it will be, because more than likely you'll want to make the patches smaller than they really need to be and you'll end up welding to metal that isn't perfect. Which makes the welding a ton harder. I've made my own share of patches, and the more I do it the more I agree with replacing entire panels. It may seem harder, but when you really start considering the time, effort and amount of welding to make and install smaller patches it tips the scales.
I'm not a factory correct restoration guy, I'm also a guy that doesn't care if it looks perfectly original as long as it's structurally sound. But honestly, if you're gonna replace more than about 25% of any given panel, you're time, effort and ultimately money ahead to do the whole panel. It costs more up front, but in the long run when you consider time and materials it's easy to end up piecemeal'ing your way into spending more money in the long run. Been there, done that.