Can i add more freshwater invertebrates or am i overstock?

i'd say your a bit overstocked. a good rule of thumb is to keep a larger number of a few species. corys should be in a group of 3 or more. also otos should be in a group of 3 or more. And tetras definitley need to be in a group of about 6 or more. I say leave the tank how it is and hopefully those fish will be ok.