I really think that CO2 induced pH changes are very different to those caused by other dissolved substances, after all a swing of pH when you turn CO2 off or on doesn't seem to effect the fish at all. I wouldn't want to try adding enough humic acid (or otherwise) to induce a 1 degree pH swing in an hour or so!
I always assumed adding anything would increase the TDS, but apparently the use of certain acids, such as those liberated by peat actually lowers them; I don't understand this completely, yet - but I'm working on it!
My breeding tanks are pure RO water and with peat a pH of 5.5 isn't too hard to maintain. I think the main area for extra complications (especially for you) could be the TDS (if that is critical) as I have found that one thing that really ups the TDS is adding dry fertilisers! The TDS in my cube with killis in went from a TDS of 80 (when just using RO water and a small amount of RO Right) to over 350 when doing EI with over 50% weekly water changes! I'm going to find out the hard way when the new fish for my Rio arrive - I have 4 wild Biotoecus sp.'Tapajos' arriving next week and they need very, very low TDS to breed successfully. If they are happy with the ferts then I think the Nanochromis should be fine.