In my country, we used to put nothing under rimless tanks. I'm not sure if this will work for earthquake-socal-area.
However, I think you should do it, especially for rimless tank. We are in socal too.
Call few more friends, 20g is not a big deal. Cut it first, drain more than half water in it (a 20g with substrate and 5-7g of water is easy for 2 and a third person for putting the mat in), lift the tank up and put the mat or HD foam under it. So you don't stress out your shrimp.
Good look with your setup, I really like it. Learned many things from you.