So you've changed from a 10 to a 20 gallon shellie tank? Excellent!! That gives you lots more room for 'scaping. I am boxing up some THR for you right now, and the pic of this tank gives me a good idea of which rocks to send.
Describe to me, please, the layout you are hoping for; rocks on one side, or both ends, some in the middle to break up sight lines, or a combination of that?
I know shellies make their homes in the shells and like to dig around in the substrate, but will they want to swim through holes in the rock? I don't want to send anything with holes they might get stuck in.
I know what would help, draw a sketch of your dream tank and upload it, if possible; I'd like to try to help make it happen.