I might be wrong in my approach, but I think overstocked is ok IF you have a heavy planted tank, good clean up crew, good filtration, and the expertise to know your tank well. I have kept 10 gallon tanks in the past, heavily planted and filtered, with about 20 guppies (+ dozens of babies that I gave to a friend with a 100 gallon), 2 mystery snails, and countless shrimp. I propagated houseplants in the filter (begonias, spider plants, and pothos mainly) and had 2 huge pothos plants rooted in the water column. I never had any issues with ammonia, nitrates, or nitrites and my fish lived very long lives. I don't stock that much into a small tank anymore, but it has worked.