Wow... I thought I did plenty of research.
I've read red cherry (neo) has wider range of pH and crystal red (caridina) has narrower range but they overlap.
I thought the main reason to use buffering substrate is keeping pH in check. Why it can't be used for neo? I've read multiple place suggest using buffering substrate for neo too. I'm not questioning you. I've read few threads you participated and seems like you have great deal of shrimp knowledge. That's why I hope to find out the reason from you. Is that for perfect or good enough parameters?
Yes, cherry shrimp do have a wide range of parameters. However, generally speaking, they thrive better with a KH at, minimum 2-3 with a pH of 7.0-8.0 range.
Most of the fancy Caridina/Bee shrimp do better with a KH of 0-1, max of 2, and a pH in the range of 5.8 to 6.8.
If you put Caridina into Neocaridina parameters, most often they will not reproduce. They might live, but they wont be thriving.
You put Neocaridina into Caridina parameters, and you might get *some* breeding, but they may not breed as well as the Caridinas. Neos mature faster than Caridinas do, and they produce more eggs, to my knowledge.... so if Caridinas are out-breeding Neos, that may say how 'well' they like those parameters.
I would say that Tigers are the exception. They can live in either Neo or Caridina parameters and, if set up right, will thrive in either condition! I have YKK's (TT mix) that were raised in Caridina parameters and living in the lower end of Neo parameters. I've had them since April, I have a bunch of babies in the tank that are a little over a week old, and several berried females. In the short amount of time that I've had them, they've been doing far better than the Neos I've been keeping for over a year now!
If you want to keep Neos and Caridina in the same tank, then my recommendation is to be careful who you purchase from. Do not buy imported Neos. Do not buy from anyone who's keeping Neos in Neo parameters. Do not buy adult shrimp. For the best chance of success with this tank, find someone who has been raising Neos in Caridina parameters for several generations. If you do it this way, they'll be better acclimated to what you want to keep them in, which means a higher chance of success for them to thrive.