Lisa's right on the money; it sounds to me like you're maxed out on dwarf cichlids for a 29-gallon tank. You could add some dither fish (small tetras or Nanostomus spp.), however. Given the preferred temperatures of your two Mikrogeophagus spp., one species might thrive at the expense of the other: M. ramirezi prefer warmer water than M. altispinosa.
For info about other dwarf cichlid species, you could pick up a copy of Linke & Staeck's 1994 book (out of print) or Romer's atlas. Lisa should be able to help you get them.
Have you fed your fish a shrimp today?