Using a calcareous substrate is definitely much, much better, safer and less expensive that resorting to chemicals to adjust water parameters. And the chemicals may not work anyway, for various reasons.
As to the substrate, rather than mixing these, consider one on its own, and for this with rift lake cichlids I would vote for something like CarribSea's Eco Complete Rift Lake Chichlid Substrate: Eco-Complete Cichild
Pure white substrates are somewhat stressful on fish, plus they add an unnatural brightness to the substrate which makes viewing troublesome, at least for me. The substrate in any fish tank is going to be more natural if it is not obvious. The above product seems, from the photos, to be natural. And it is calcareous.
Crushed coral on its own, in a sand grade, is similar but it only adds the calcium; magnesium is still missing. An aragonite sand would have both. As does the linked substrate.
Try to see these "live" if you can before deciding, but if this is not possible, use the photos. There may be a cost difference between them.