The limestone rock stuff and substrate thing is something that definitely won't hurt in possibly helping to sustain higher PH and water hardness in your aquarium. But, I'm not sure how effective it will actually be though, esp. if you wind up doing a lot of high-volume water changes to dilute Nitrate buildup in a heavily stocked tank.
Crushed coral is used a lot with former reef or salt tanks. It is a bit pricey. But If you're set on it, I would get it in as small diameter particle size as possible. Your cichlids will dig in it and it's easier for them to work with in a smaller size if any is accidentally ingested. Personally, I prefer and recommend Pool Filtration Sand (PFS). It's cheap, won't cause any problems for digging cichlids and looks very natural in the aquarium.
But, to each his own I guess....
As far as rocks go. You need a good sized pile(s), esp. for Mbuna type cichlids. Build your rocks up all the way to the surface, but leave open expanses of sand for your males to have room to do their thing. Big rocks are better than a bunch of smaller ones, because they stack better together and are more stable in a pile. Safer. When ready, build the rock pile(s) in the tank first. That is, place the rock directly on the glass, and add the sand afterward. This will keep digging cichlids from possibly undermining the rock pile later, and accidentally causing collapses or dangerous rockfalls. To purchase rocks, local garden supply centers and plant nurseries may have the cheapest stuff. I personally like Mountain Lace Rock because it looks gnarly, comes in big pieces and stacks very nicely together.
Check your tap water for General Hardness (GH) and PH. If you have a natural source of pretty hard water that is decently high in PH? You may not need to add anything at all to the aquarium water to raise GH or boost the PH. :wink: