Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
I had both Prankster and Mycelium Might active at the same time in Random Battle Mayhem, and Toxic did not work against the opponent when Comatose was active. I think that Prankster just completely overtook the effects of Mycelium Might, unless for some reason Mycelium Might does not go through...