Are emulators valid or at least is there a emulator category? Which are the recommended ones? Is Mesen ok?

Emulators are allowed, and it's in the same category, it just has a label next to it saying that the run was done on an emulator. In fact, all of the top three runners use emulators, and I will as well once I upload a run. I don't know the specifics of what emulators are recommended and I don't know what Mesen is, but I think any emulator would work fine.