You can silicone lava rocks together to make them more stable. Or if you have a really nice and big piece, you can grind it on cement or other abrasive until the bottom is flat.
Could you explain epoxy combined with super glue I'm assuming using them separately expos for the rock and superglue for the plants correct? What epoxy would you recommend?Epoxy combined with super glue makes a strong attachment. Also placing eggcrate on the bottom of the tank before adding substrate is a good method for placing your rocks, instead of directly on the glass.
Is the lava rock found in the local yards safe to use? Is it the same as the stuff found at the lfs?Lava rock and silicone is pretty stable when it sets up. You can't transport it or anything, but it will stay together inside a tank without issues in my experience. I would find a larger, single, rock and use egg crate though. Local rock yards are a good source of decent stone for a fair price. You can hammer away or use a grinder to get the desired shape and such as well.