Everything else gets more complicated. 2" styrafoam is nice but do not put sand over it. There are many reasons but it is just not a good idea. When the walls are up and the base as smooth as possible lots of time and duct tape can be used to install the foam. A lot more time and duct tape to install the pool cove. At this point the cove is optional since you already have the bottom rails and any sharp edges well covered.

The foam does provide a very nice base for the liner. Nice and easy on the feet.
I'm confused. How is the cove optional? You mean I don't have to use pre-fab cove OR sand? And when you say the foam does provide a very nice base for the liner, do you mean styrofoam? Instead of sand (since I can't put it over the sand?). I'm sorry, I'm just not understanding what you're saying. If I'm trying to pinch pennies where I can, what do you recommend for the following:

base under lining (remember my rocky soil)
wall foam (I mean lining the sides of the pool) or not?
pre-fab cove or sand cove or any cove?

Thanks AGAIN!!! You don't know how much I appreciate this!