First, your PH sounds right for Southern California tap water, but your GH does not. GH for most Southern California water sources should be much higher. Here in Orange County, my GH is >10 and my KH is sky high as well. TDS varies throughout the year from around 280 to as high as 500.
Now to address your questions:
1) The content of tap water can change throughout the year as your local municipality may switch water sources at any time with no notice. Often water is pulled from 2 or more sources depending on the time of the year. This makes it hard to maintain consistent water parameters.
2) The chemical treatment used in the water may change throughout the year.
3) While Aquasoil or similar will help lower PH, by starting with high PH/KH water, your Aquasoil will lose its buffering capability more quickly necessitating changing the soil more frequently. Also, my experience is that Aquasoil won't take PH 8 water down to 6.5, or if it does it takes a long time.
4. Even if you get the PH right, your TDS is likely to still be too high for the shrimp.
Neo's and Amanos do great in Southern California tap. CRS and other bee's usually do not. I know of some who have managed to keep them alive in tap, but don't know of anyone who has gotten them to breed in 100% tap. There are however plenty of people who mix aged tap water with RO to get the correct parameters for their shrimp, rather than using supplements.