Very likely your best option is to sell all your current tires and also the Fiero wheels, and get new tires (rated 41 psi or higher) that are the right size for your Neon. More notes on that further down below.
People, people... With all due respect...
The question is not so much about the relative merits of higher pressure, but which tires + wheels should HypermilingNoob use on his Neon? You've seen me discuss high pressure tire use here plenty, but that's not the biggest issue here.
First item to address:
What is the stock tire size and outer diameter for each of these cars?
HypermilingNoob knows they are 14" and 15", but that's only the rim diameter.
Tire outer diameter is what "drives" the final drive ratio (I use the term loosely) and is also what keeps your speedometer/odometer accurate. A tire of larger outside diameter will of course give you lower rpms HOWEVER they may not fit your wheel wells, especially in curves or on bumps when the springs compress. And, personally, I prefer to stay with stock diameter to keep the speedometer/odometer accurate. True that you can use GPS as a substitute but only if you have one, but HypermilingNoob may not have one.
Anyway, find the tire sizes imprinted on the various tires you have. If the Neon's four 14" tires are NOT all the same size that's a good reason to ditch any that aren't a match. You should have matched pairs front and rear, at least. Most would say that all four should be same make/model, and of course should be same size.
Knowing the sizes of all your tires, use this calculator to see if the outer diameters are the same:
Tire Size Calculator
Likely the Fiero tires aren't the same outer diameter as the Neon but go have a look.
Your Fiero wheels may or may not fit the Neon. Use TireRack.com as a resource to figure out wheel fit specs. If you can't make sense of it, call their 800 number and get them to tell you. Best option for the Fiero wheels/tires is to sell them to a Fiero driver. Craigslist.
Here's another tip: Even if the Fiero 15" tires have the correct outer diameter they may be wider than the Neon spec size. If so they might still fit in the wheel wells but maybe not. Further, if wider they likely will be heavier. Heavier tires hurt FE, all other things being equal (which of course they never are). You can use the "specs" references at TireRack.com to learn about tire weights. Find a specific make/model in your size and find the weight. You can get an idea of how weight changes for different size tires by scanning the chart.