my ten gallon mantis tank so far!

It depends on how large the rocks are. My mantis tank has the rocks sitting halfway in the sand because my mantis doesn't burrow all the way under, she uses the rocks more than anything as her den. And the rocks are small enough that if anything did tumble, it wouldn't even hit the edge.

Looks like a good start to me! Plus, burrying rocks with a mantis is kind of in vain. The bigger ones will reaquascape for you no matter what you try. So if you end up buying a young peacock and letting him grow up in that tank, he'll move your rocks around for you

 

I have all my rocks in my mantis tank on top of the sand. The mantis will try to dig under them and will probably strike the bottom if the rocks are sitting on the glass. You should have a piece of acrylic on the bottom for that reason though. I don't have any rocks stacked on top of each other so I don't worry about a rock slide.