Laetacara dorsigera, Laetacara curviceps, Microgeophagus altispinosa (Bolivian Ram) would all be good choices for a 20 long. If set-up correctly you could do two pairs of any of these fish in a 20 long.
I wouldnt do the cutteri, multispinosa, sajica in the 20L. I have all three in my 300 which more than enough room but a 20L would be a tight fit in the long run as stated previously. Go with apistos