I would think that kribensis (Pelvicachromis Pulcher ) would be more adaptable to hard akaline water than the south americans, and are certainly more forgiving of fluctuations in water quality than rams or apisto's. They also might not be as easily disturbed by the antics of a growing RTBS as apistos or rams, however you will proberbly be pushing the limits with very hard alkailine water. It may be best to get the C02 running for a few weeks, and get the hardness tested and see what you end up with, as its hard to say how much effect the C02 will have without knowing the kH of the water.
I would say that rams (Microgeophagus type) are actually quite fragile, even though I personally have no problems. They do not seem to tolerate ammonia/ nitrite spikes even at small amounts, and require low levels of nitrate and dont appear to adaptable to unatural water conditions. This may be due to the quality of the stock out there though. Tank bred apisto's such as Cacatuoides seem to be more forgivinggiving, in the fish I have had contact with.
As jerry mentioned, mixing cichlids can always be a bit hit and miss, especially if they spawn in there. You might get away with 2 pairs of apistos or a pair of rams and a pair of apisto's in there, but I wouldnt want to mix kribs with anything others in a tank that size, as they can be a little over powering.
Dependant how hard your water is, fish from the african lakes may find the water chemistry more appropriate