I agree with what has been suggested. The only cichlid options for a 20g long are either shellies if you have basically moderately hard or harder water, or dwarf South American species if soft to moderately hard. The pH at 7.5 is pertinent, but the hardness is the real issue. You can (or should be able to) find out the GH (general hardness) from your municipal water supply people (check their website) unless you are on a private well.
If you go with community setup, and depending upon the GH (some of these are more specific), many of the "nano" species which are becoming popular are more than suitable. Being so small, you can have more fish/species in a 20g long which adds interest to the space. But there are also a number of other more standard species that would work for you too.
Byron.