They look to me like bits of limestone in a sedment matrix so I'd call it a breccia. And I personally don't much like the Rena heaters as they're a bit bulky though they've always been reliable for me.
I've kep a number of africans in 20 longs before, which are about 30 inches long I think, but that's a 20 high and long term, or even medium term I think it'll struggle. Thsi has all been said before.
Controlled overstocking works, but only to a point. It works because there's so little space noone can setup territories, and there's enough space for a chased fish to run and hide. However in a tank of this size, there isn't enough space for a fish to run. That's your problem, even at this size, and these fish will grow, no doubt about it. It's nothing to do with water quality, it's just about behaviour.
I have kept mbuna in 20 longs, but only the smaller, less aggressive types, and even then I have a lot more rock work - effectively there is only cave in this tank as the auratus will want the hole rockpile as it's territory