Well in the wild they don't come from the same area and/or water type. And many African cichlids are quite aggressive.
There are a lot of horror stories out there of plecos being ripped to shreds by Malawi cichlids. It's not pretty. Some people have had a bit of luck using the barbed types, but in general it's not recommended (at least those I have talked to about it in the past) to have plecos with Malawi cichlids.
There are even stories of plecos just rotting away (fin/tail rot) not long after being introduced to the tank. Something about pH?
However I have heard rumor online in fish forums that some plecos can live, even breed in tanks with Lake Tanganyikan cichlids. Just another lake in Africa with slightly different cichlids.
The only way I could keep my 50 gal tank free of algae was to manually scrub the glass every few days and then let the external tank filters take care of the rest. If it gets really yucky, you can do a partial water change.