I'm borrowing/modifying bits and pieces of the anime and Pokemon Tabletop for my power/scaling system.

One thing I wonder though, is what to do about "environmental" moves. For example, should a Pokemon be allowed to Sand-Attack in a clean hallway that has no sand? Same thing for moves like Mud-Slap, Surf, Rock Slide, etc.

Do most semi-realistic combat systems just disallow a Pokemon from using those moves, or do they allow Pokemon to "manifest" things like Sand/Mud/Water/Small Rocks?

The videogames implicitly do the latter, but there is some precedent for the former with abilities like Desolate Land, where the harsh heat/drought just causes Water-type attacks to fail completely. Can't use Surf if all the water is dried up.