pH and hardness will determine the right choice basically. If you have hard water, nanoluteus like duanes suggested would probably be the best CA cichlid for your tank. I have had success having a pair of HRP/convicts in 20 gallons in the past. I also just tried to keep my panamensis in a 30" tank to get them to breed, and the male beat the female. So it's hit or miss with cryptoheros types in a 30" tank IMO. But the nanoluteus is the smallest of them, and a bit less mean, so a good bet.
With softer water, or even just with neutral-ish stable water, I think the cacatoides you posted or keyholes would be great choices. The ram would work too but personally I don't like the "balloon" fish or other mutations like that.