Basics of 1/72 Diorama's?

Coming from the model railroad angle, I think the key thing is exactly what you say: Scaling it down. Model railroaders are famous/infamous for experimenting with unusual and off-the-wall materials to achieve some quite phenomenal results. When approaching such a wide variety of scales, I have always enjoyed studying RR modeling magazines - most notably Kalmbach's own "Model Railroader," but also "Model Railroad Craftsman" (ducks to avoid a rock tossed by forum management). If any are masters of scenery, these guys definitely fit into the "world building class" as such. I can't count the hours I've spent reading several boxes of magazines given to me by a veteran modeler – ogling over phenomenal photos of entire cities and vista-like landscapes, just soaking up beautiful work on many scales, without even making any efforts to actually accomplish anything beyond laying some track on a board!

Much of the material for diorama scenery is useable on varying scales - such as for trees and shrubs - just choosing smaller sizes of the same. When thinking in terms of grass and smaller details of course other materials must be considered. Ultimately, it is what ever looks best regardless of what the norm is.

Always keep in mind the end result foremost, and then compare your intended modelling material with the desired goal. Your eye is what needs to be appealed to most, and not just "what others think."

for the few dio,s ive built i can only say this, if it looks right use it , try using the same technique and if it looks ok to you then it probably will to others if not adjust accordingly, ive used some of the same things in my 1/35 dio,s as my 1/72 bases usually the technique stays the same just down scale the materiels a tad.