No need to stick with one method. This scene has rock castings, carved Styrofoam, and Sculptamold rock strata all on the same cliff. Structolite was used on all but the rock castings to add some texture with that perlite you mentioned.
IMO using a variety of methods adds interest.
The pre-colored rock castings you made look really good. I'm going to have to try coloring the plaster first on a future project.