Well you should realy keep corys in a group of at least six, but with a 72 litre tank already holding 10 tatras I dont think you will manage it.
You'd be fine to add 6-10 Corydoras, I'd be tempted to go for one of the 3 dwarf species: Corydoras pygmaeus, Corydoras hastatus and Corydoras habrosus.
