Totally agree if they are small juveniles you are ok but when they grow up you will definitely be overstocked if you have other fish in there. One inch of fish per gallon is a good rule but some fish need more territory then others.
Convict 4"
Jewel 6"
Firemouth 7"
Total of 17" plus whatever else you have when full grown.
I have a 30 gallon and I don't think I would want them in there because lack of swimming room for a 7" fish in a 20 gallon. I like Dwarf Cichlids for the smaller tanks like rams, cockatoo, macmasters or any other Apistrogramma because they are small and cool looking.