1.) Don't use tap water on a buffering substrate. This can cause unstable water parameters, which shrimp don't like, and wears out the buffering capacity sooner. *IF* you want to wear out the substrate and not worry about parameter swings, then by all means, keep using tap and add more KH! Otherwise, switch to RO water and GH only minerals.
2.) If you don't have them already, use liquid GH/KH test kits and get a TDS meter so you have a better idea of what your water parameters are.
3.) I have a tank currently set up with SL-Aqua soil, RO water and GH minerals. pH is about 5.6, GH should be around 8 with a 150+ TDS. KH is at 0. I have Neos in the tank. I honestly didn't know if they'd survive or not, since the tank was previously set up with sand, RO water and GH/KH minerals at 8/4, respectively, TDS 150-ish. I just did a 100% water change with new water and switched substrates, then plopped all old inhabitants back into the tank. Not sure how many Neos are in there, but probably a handful or so.