I'm not exactly one for measurement, for aquariums or cooking...LOL. It depends on the parameters you're starting with anyway - you have to play with it to find what works for you.
You can often find Tufa Rock in fish stores. It's attractive and great for plants that attach their roots to objects. I have varying sizes of Tufa, Texas Holey Rock and seashells scattered in all of my aquariums.
The crushed oyster shell is great because unlike coral, it dissolves so you know when it's depleted. The small pieces dissolve more quickly than larger objects as well. I put that into my HOB filter cartridge, box filters or in a media bag.
The coral with aragonite or aragonite sand can be used as a substrate, but it does lose it's buffering power eventually.
I also add a dash of seasalt and calcium montmorillonite clay with every waterchange. Just enough to add minerals, since fish absorb minerals from the water column as well as the diet.
Guppies can be highly adaptable (unless they've been improperly bred to a point that they can't cope with anything). The fish born into your new water parameters should thrive, even if your current adults continue to struggle. I've successfully kept all of the species you've mentioned in my current water parameters, if that reassures you at all.