For a tank that small, I would think dwarf species such a Nanochromis, Pelvicachromis or maybe some of the Pseudocrenilabrus like nicholsi or multicolor might be some of the most suitable cichlids.
I was about to say one of the smaller and mellow Pelvicachromis like roloffi, subocellatus, or taeniatus. I keep a pair of Enigmatochromis lucanusi in a slightly smaller foot print with some Norman's lampeye killifish.