Believe what you wish, the fact of the matter is that the African Rift Lakes are very different from the tropical rivers and lakes of south/central america. Not only are they different as far as water paramaters goes(Lake Malawi pH averages from 8.5-8.6), but the type of habitat in each place is quite different. If you were mixing African river fish(ie: kribensis), I'd say it's cool. There are some Central American cichlids that can tolerate higher pH but the hardness of their water is different.
See these:
http://malawicichlids.com/mw01011.htm
http://www.fishkeeper.co.uk/hr/lake_malawi_cichlids.asp
Not a single African rift lake has an average pH below 8.0.
Perhaps the fish can live just fine in subpar water conditions, but to see them in their optimal coloration/health, proper parameters and habitat conditions should be met. Mbunas live in rocky habitats, SA/CA fish take refuge in butress roots of trees, fallen logs, plants, and some rocks (albeit different type of rocks and geological formations).
Both SA/CA and African Cichlids have lived in different water conditions for thousands of years and their gentics have evolved accordlingly. To throw them in a tank of sub par water conditions that are condtrary to their genetic evolution and expect them to live perfectly is just asinine.
If you're not interested in proper fish keeping, go for it, keep mbunas with firemouths.
