I would be very careful using a heating coil/matt with frogs that dig into the dirt or can touch the sides of the tank; they can easily get burned. I am not sure about the temp requirements for a pacman, but I know using a heat lamp may also be too warm unless you can get it just right, and it may dry out the setup quicker, which means more mistings. Pebbles would be okay as long as they were not small enough for your frog to swallow. Most frogs have a "try to eat it first, spit it out later" attitude and it could end badly for your frog. Best of luck!