Several things here, starting with the pH. It is the GH not the pH that matters, as this is the source of the calcium snails require for their shells. As for pH, a difference of a couple decimal points is not going to harm the snails, so I would not worry. And a pH below 7 will absolutely make life a tad easier for the named fish, though again this is minimal, but not worth bothering about.
To the tannins...this is not likely to be what the BH need, as they are not blackwater fish (nor are the cories). But I am assuming you are thinking of light issues, and the BH are found in dark or dimly-lit sluggish streams in forested areas of the upper Amazon basin along the borders of Peru, Columbia and Brazil. So floating plants would be the best remedy here, as these replicate the shading effect of overhanging marginal vegetation which is common to almost all forest streams and creeks in the Amazon basin. Few of these watercourses ever see direct sunlight.
Obviously the snails won't have issues with less light, probably enjoy it too!
