Yes on the 20g high being OK for the pair of rams, provided they are a bonded pair. Not all males accept all females and vice versa. I can explain further if asked.
Definitely increase the corys though, another 3 for a total of 5 (assuming you have one of the average-sized species). This will settle the existing two and have less effect on the biology than otherwise. And if you have live plants (floating if nothing else) and are regular in weekly 50% water changes, you should be fine.
One comment though...if you intend breeding the rams, meaning raising fry, I would not have corys or any other substrate catfish. These are nocturnal and will easily eat eggs and fry in darkness.
Byron.