It would be considered hybridization of morphs, and these morphs may actually be found to be different species or subspecies, and you may thus have hybridization. Ask the original discoverer of the Endler's Livebearer what he thinks of the little fish as it is seen in the hobby today--he'll see that it is not the same due to people thinking that these were just guppy colour morphs at first...so, in summation, don't let them cross, especially if they are Rift Lake cichlids, because we don't know enough about the ichthyofauna to know if they will be classified as different species later!