To me, the rocks are the best way to go because it would mimic what mbuna see in the wild in lake Malawi. If you look at footage of the lake it's basically just a rocky bottom. Another way you could imitate this would be to create a 3-d background yourself with lots of small caves and overhangs. You can use expanding foam or Styrofoam to do this, there are plenty of methods. Quick youtube search will bring up a bunch of tutorials. You could also use some tough-leaved plants in the tank. Mbuna will pick at most plants but there are some which can hold up to it well, I've had good luck with anubias and crypto sword plants in mbuna tanks.
On the hybrids, it may be an interesting concept, but please don't let the hybrids leave your tank if that happens. Don't sell them or give them away. Most will probably be eaten by the mbuna, and you could also add synodontus catfish (like s. petricola for this tank size) which live with african cichlids in the wild and will also help eat the fry.