No corydoras in a regular 20 gallon. 20 long is minimum. And as for rams, you can breed rams in a 10 gallon but not keep them in one. Only temporary.It will only work if you have a male/female mated pair. Even then during spawning they can be aggressive towards other fish. I ordered 2 boliviam rams online and wound up with two males. The dominant male claimed most of the tank as his territory and the less dominant one was forced into hiding and being chased constantly. So I have the less dominant male in a 10 gallon until I get my 55 gallon is up and running. Which I would say is about the minimum tank size for to have two bolivians so each can have their own territory. Otherwise you can definitely do just one bolivian in a 20 with a school of tetras, and a school of cory cats. More than one ram in a 20 gallon in my experience is asking for trouble.
Which is why I mentioned it's in my 10 gallon "until my 55 is set up".....and yeah on corys going in 1 20l vs 20t. OP is your tank a 20 long or tall?No corydoras in a regular 20 gallon. 20 long is minimum. And as for rams, you can breed rams in a 10 gallon but not keep them in one. Only temporary.
In that case I think 1 ram and a school of tetras would be bestThe tank is the standard rectangular shape. More long than high.