Hello and welcome,
Just a first quick help.
1. Please do not use peat extract from garden shop unless you make sure it does not contain any additinal fertilizers or other substances. It is very likely it will not help you adjust the pH, I would assume that commercial peat extract would be buffered so that it is not too acidic. That of course means it would be useless for bringing the pH down.
2. Humic substances will not harm the parasites, but if you use the right humic substances, they will of course help the fish, lower the stress and help heal the skin. The lack of zooplankton in black acidic water is mostly caused by the lack of bacteria in the first place. In case you will be forced to treat the fish, avoid using medication that contains malachite green in acidic water. The toxicity goes over the roof. I would try to use quinine based treatment. Also if you use something that would truly contain humic and fulvic acids or their salts, keep in mind that these humic substances really influence bioavailability of many substances and for example pesticides administrated with addition of lignohumate have the same effects with much lower doses. The same may apply for the treatment. I would like to help more but my Paro experience is not extensive enough + even thought I have pretty extensive experience with other black water species I never had to treat any of them for external parasites. Most of my other species are extremely susceptible to commonly used treatments and I just have to be very careful to not get in such situations. Also “humic substances” are super complex topic and we know very little about them, besides their positive effect on plants and animals. I think Peter and Bernd will be able to give you more practical advice with regards to possible treatment.
3. In case you want to have other fish with Paros, go for some really black water Boraras species, uropthalmoides are not the case and you keep both fish in borderline conditions. One more advice if you do not mind. Never trust any fish or plants no matter how famous and known is the breader. As it is with humans, some populations are highly susceptible to certain viruses, bacteria and parasites, some are not or not nearly to the same extent. The fact that you have not seen fish in the tanks does not mean that they were not in there short time before and these do not have to be fish, for example snails are carriers of many diseases transmitable to fish. Quarantine everything always.