It's not really recommended to combine Mbuna and Haps. The main reason is dietary requirements. Mbuna are almost entirely vegetarian where Haps need meaty foods. If Mbuna get too much protein they can't digest it properly and fall victim to "Malawi bloat". Unfortunately they will quite happily eat the foods that cause them problems which is why they are best kept separately from Haps.
A larger rockpile (firmly fixed) will spread the Mbuna through the tank as they take up areas of territory.
A lot of people put egg crate on the bottom to prevent damage from accidental falls and increased pressure. Then it's a case of spreading the load evenly so the weight is distributed across the bottom and not too concentrated at one end.
When you think about the weight of water a tank has to hold, the rock makes little difference as long as the load is well balanced and the tank is supported as per manufacturer's instructions and properly levelled.
You can reduce the weight in Rift Lake tanks by using some Ocean Rock, which is actually limestone and full of holes and is less dense than solid rocks, or Lava Rock which is also lighter than the same volume of rocks like slate.
Corals and limestone act as buffers and while in a low pH environment they will raise pH, their real function is to try and keep an equilibrium. Above a certain level they no longer dissolve so there is a limit to how high the pH will go, and the higher the starting pH the larger the amount of additional carbonate that is needed to effect even a minor further increase. As pH approaches 8, the rate at which carbonates from decor dissolve into the water slows down drastically and the amount that needs to dissolve to raise pH further increases drastically. In effect this means pH will be unlikely to rise in a tank with a starting pH of 8 that has a large regular weekly water change as is needed for Malawis, and if a rise occurred it would not be likely to go beyond 8.2. This should mean very stable conditions can be maintained.