In all honesty I would think smaller than a Convict. Males can with care and time grow out to 6in or better. That would be a tight fit and look as silly as 12in Oscar's in a 55gal.
I would look for smaller members of the Amatitlania family like Nanolutae or Honduran Red Points. The males only grow out to 4in so relatively more space that the Convict.
As far as dithers go, to be more bio graphically natural go with a live barer like Swordtails or Plattys.
Or a nice tall bodied Tetra like Black skirts, Buenos Aries, Bleeding Hearts etc. Smaller Tetras could be late night snacks, as i have learned first hand.
Adult male HRP
Adult male Nanolutae