Crystal shrimp do best in a tank with 5.8 to 6.8 pH, with a KH of 0-1. The easiest, and most stable way of attaining those water parameters is to using a buffering substrate using RO/DI water and a GH+ only mineralizer.
It is possible for them to live in the lower end of Neocaridina parameters, but they may not thrive as well in higher pH with higher KH levels.
Using products like pH Down or Discuss Buffer is just asking for a whole host of problems and quite possibly the death of the shrimp! Products like those may raise the TDS while lowering the GH (which shrimp need), and if there's nothing to buffer the water to those parameters, then the pH may rise right back up into the 7.0 or higher range. Dose the tank again, you raise the TDS yet again, possibly even lowering the GH further (re: molting issues), and the pH is going up and down like a terrible roller coaster....
Alternatively, you could try using things like driftwood, indian almond leaves or alder cones to lower the pH, but then you need to make sure that all new incoming water matches the tank water to reduce the chance of the pH changing too much... and these things may release tannins, causing the tank to be a brown or tea color... which might be less pleasing to the eye.
Bassically, if you want the best chance of success with crystal shrimp, then start out with a 10 gallon tank using buffering substrate, RO water and a GH+ only remineralizer.