We are having an inground gunite pool installed with an inground acrylic 88" spill over spa that will be surrounded in flagstone and coping. Should the spa be placed on a layer of sand or gravel instead of dirt? I am now also worried about the flagstone and coping (that will be placed next week) sticking to the slick surface of the acrylic spa. Any suggestions for how the flagstone and coping should be placed?

Welcome to TFP!
Whatever the spa is placed on needs to be completely compacted; crushed stone is best. I'm not sure about the stone adhering to the acrylic; I would think that the installer would be familiar with whatever is needed to make sure the mortar sticks. Sorry I can't be more help!