Yeah, you can go to Home Depot and pick up some egg crate to lay on the bottom of the tank to disperse the weight of the rocks. Then you can put your substrate overtop. I suggest you place your rocks first, then your substrate, as some cichlids love to move the gravel around and could cause a rock slide.
Another thing to consider is that Sting Rays require maximum ground area, whereas the cichlids will require a lot of rockwork and other features that will use up a lot of square footage on the bottom of the tank.
(oh, it's spelled Malawi
)
Good luck!