Captain Lee's Auto Spra-Strip is what is recommended for fiberglass Corvettes.
I just used it and it dissolves the paint but not the primer. And the Corvette I just did was a '69 and it still had the factory primer on it. So it didn't even get to the fiberglass. I sanded the primer off afterwards. That was still a lot easier than sanding off the 2 layers of paint that were on top.
It is not supposed to eat into fiberglass at all.
Don't know about using it on urethane or plastic parts though. But if it doesn't go thru the primer, it should be OK on that too.