This depends on the species, if it is the smaller species yes they like company, if it if the larger species then it will reach 8 inches (eat everything it can fit in its mouth) and prefer to be alone as far as others of its species.Once again just because he works there, it doesn't mean he knows what he's doing. As for you stocking Upside down catfish do best in a school of atleast four and cories need to be kept in schools of atleast six. The tetra also need to be kept in a school, atleast six, maybe some more it depends on the species. Can you provide a picture? The Neon fish are either berry tetras or Glo- fish( Zebra Danio) The pleco is most likely a Common Pleco which grow extremely large, soo I would return it. Click shaded names to vist profiles.