Any switch connected to the ERR RailSounds Commander chuff input MUST BE TOTALLY DEDICATED to that task. It's not possible to share that chuff switch with anything else. The circuit on the RS Commander does not have a common DC ground with the rest of the world.
I you really want chuffing smoke and synchronized sound with a fan driven smoke unit for TMCC, consider the Super-Chuffer II. It's purpose built to do that job and it does it very well. Here's some videos with folks that are installing the Super-Chuffer.