I find seriouslyfish.com often has good info on specific habitats including decor, plants, and congeners (species they share habitat with) but not many sites seem to have good info on fish that live in the lake that diamond tetras live in, as they are endemic to lake Valencia in Venezuela. According to the World Lake Database, Valencia seems to be home to a few species of cichlids like Mozambique tilapia, Petenia kraussii, and Geophagus surinamensis, and some odd eels and catfish. There's also apparently Poecilia sphenops which are a species of molly. I would probably stick to what CorydorasEthan said, probably much better options for a peaceful community haha.