I agree with mommy1. I have not myself kept this fish, but according to the info on the cichlid site, when kept with other fish they need to be robust and hardy in nature, though not large enough to eat this cichlid. I think that rules out most small fish like tetra.
Your alternatives are better. Some info on the Keyhole from my profile of the species, Cleithracara maronii:
Origin and Habitat: Rio Orinoco basin (Venezuela) eastward through Suriname, Guyana and as far as the Rio Ouanary in French Guyana; possibly Trinidad. This species is not abundant, occurring in small populations within this geographical area. It is found in slow-moving, shallow coastal forest streams that are often stained brown from the tannins of decaying wood. The population reported from Trinidad has not been encountered since 1965 according to Kullander.
Compatibility/Temperament: Very peaceful except when spawning. This is a very shy and retiring species, easily frightened [see additional discussion under Description] and should not be combined with other more aggressive cichlids. Peaceful characins, rasbora, and substrate fish are ideal as "dither fish" to lessen this species' natural shyness. In larger tanks, it may be combined with discus, angelfish and some of the dwarf cichlids.
Description: An ideal cichlid for a community aquarium of non-aggressive fishes. Kept on its own, it will likely be very shy and retiring, and prone to stress. The presence of other fish (termed dither fish) will keep it more settled and relaxed. This is certainly not a fish for a barren tank, in which it will be highly stressed.
The aquarium should have a dark substrate, with some flat stones and several chunks of bogwood, and floating plants to shade the light that should not be bright. This would replicate its natural habit, but additional plants will also be suitable especially as they will provide more cover. Keeping in mind its natural habitat, the flow from the filter must be minimal, something that will also suit most forest fish that make good tankmates. Cichlids other than discus, angelfish and the South American dwarf species should not be kept with this species to avoid stress.
When this fish is frightened or endangered, it will fix itself against a log and change its colouration and patterning in an attempt to blend in with the background. This camouflage is very useful to the fish in the wild.
On the Bolivian (Mikrogeophagus altispinosus), this fish is best either singly (one fish of the species) or as a bonded pair. You could do a small group in a 100g tank, though with the other fish I probably would not. I have a lone male in my 5-fgoot 115g and he owns the tank; there are no other cichlids though, so he is quite docile, but he still is head fish and the others know it.