Id steer clear of Nandopsis or Parachromis species, i think these will be too similar to the Grammies and cause problems mate. Id personally be choosing from Herichthys, Astatheros, or maybe smaller/milder Amphilophus genus's. These will be able to stand up to the female Grammy, which in my opinion are the Evilest fish around!! Whatever you choose, you may be asking for trouble when the Gramies spawn.
A nice pair of Herichthys Carpinte 'Vontehillo' or some Astatheros Altifrons would be nice