For a scientist, Jacob, it’s quite normal that every answer leads to new questions. That’s completely right.
1. Even such commercial products must make use of suppliers, and peat is never the same. We know of peat that leads to a rise of hardness. So I can only give the advice to test it. But since the main effect for Parosphromenus-aquaristics is the lowering of the pH, you can try out pH-lowering acids with very often a good success. I have often used the product by Sera for this aim. Certainly, with good acid peat you introduce further humine substances which are of good use. But you can have succes with acids, too.
2. I should not use that old sponge again. Wash it completely before. Bacteria need to grow accustomed to the environment, and if the water etc. conditions before were different, the result may be contraproductive, in that they die first, possibly. Therefore have a new start of that sponge filter. I use no filter at all and mostly everything does well.
3. It’s a good idea to keep a group of Parosphromenusof the same species in a larger tank to study their behaviour. But for a beginner I think it is better to keep two pairs each separately in a small tank and breed them first, because it’s difficult to manage the feeding of the young in a tank too big. Mostly fish that are not engaged in own breeding activities tend to feed on the young of others. On the other hand: some will perhaps survive if you feed brine shrimp regularly and give Parameciumor (better) rotifiers the first week after swimming out. But to find them in a bg tank is … not easy. Anyhow: For a somewhat experienced breeder your plan is charming.
4. The kit I used lately and found very useful is called “Cult-Dip-combi” and is manufactured by the famous international chemistry company ERNST MERCK in D-64271 Darmstadt (Germany). The reference number in Merck’s list is 1.00778.0001. It contains ten kits for ten tests. Look at Merck’s site in the internet and find out how to order it. Normally it is not sold in aquarium shops. Most aquarists don’t need that. They open the tap for their Platys, and that’s it.
But let me say a general thing: This all sounds rather complicated. If you manage to get hold of some healthy Parosphromenus-fish, if you use pure c.o-water or pure rain-water, if you present them their caves, make the light not too bright, shadowed by water sprite etc., if you feed them now in summer with Culex larvae, glassworms, small Daphnia, brine shrimp, Moina (or only two of all that): You will have success. Those fish are not as delicate as all this seems to indicate. I think, you will do that quite right, even without your fine filtering system and without daily controls of the water. But you should be equipped for that.