if you do put them in the 20g, which I think is too small anyways, you should have a backup tank to put one of them in in case they start fighting. sometimes even breeding pairs will start fighting and it can cause a lot of damage or even death.
If they are a breeding pair I might be wary of even adding cories. There isn't a whole lot of room for cories to hide in. At leats make sure there is plenty of low lying plants for the cories to duck into if they need it. But keep the restof the tank fairly sparse so the angels have the max. amount of swimming room.