I'd say in a 20gal you are pretty much stocked out. Convicts can be very nasty, it is said that a convict will stand up to a fish 2-3 times its size.
When mine were breeding, pretty much every 2 months....i was scared to stick my hand in the tank.
The only thing i would really even suggest is some type of dwarf pleco, even then don't make it a nice fancy dwarf that costs a lot. A bristlenose plec or clown pleco might work. But you are pretty much full with 2 convicts in a 20gal.