What should I use to set a fiberglass shower pan to keep it from giving or squeaking, thin set or something else? This is on a concrete floor.

We puddle cement, mortar or thin set under the shower floor and let it cure. Make sure you pay attention to the drain area. The shower floor should not "give" any when you step into it. Good luck, tom

There normally is some literature with your installation guide that spells out what the manufacturer wants you to use. The only addition to what Tom has said is a modified thinset which is a bit more like a heavy glue than thinset. Before you set your pan check it for level and shim if needed, check again after you set it in place.