Stocking Help

A friend of mine has a 57L aquarium with a dwarf gourami, a cory, 5 dianos and an apple snail and he was wandering if he could add a Gold Marble Bristlenose Catfish to it. If not what other catfish/pleco could he add?

 

He should add 5 more cories of the same type. They are schooling fish and should be kept in groups of 6+

I think he is already fully stocked for a 15 gallon, plecos have a huge bioload.

 

That tank is almost full but He could add 1 or 2 other Corys. Plecos need at least 80 liters

 

He is already so full that I wouldn't want to add 5 Corys. It's not ideal but they do fine in groups of 3