I have about 200 bloody mary shrimplets in a 9 gallon with my 12 chili rasbora with maybe 3 fissidens stones that didn't offer much hiding space.
Mulm should be OK with pygmy corydoras, the "good stuff" in there actually provide great nutrition for pygmy babies.
Perhaps more relevant to your question.........pygmy corydoras do tend to swim in the water column, so I'm not sure they'll consistently move mulm like your prototypical corydoras. Another pygmy species habrosus do tend to spend more time at the bottom of the aquarium.
I feel like adult pygmy corydoras will be able to eat chili rasbora eggs though so keep that in mind. Despite having tiny mouths, those chili rasbora eggs are super tiny as well.