Yes, one can have a low TA in a plaster pool if one compensates with a higher pH and/or CH. In terms of protecting plaster, all that matters is that the water is saturated with calcium carbonate. It doesn't matter whether that saturation is more from calcium or from carbonate because from a chemical standpoint it is the product of these two concentrations that determines whether calcium carbonate in plaster will dissolve (at negative CSI) or will form scale (at positive CSI).
The TDS, so salt level, also affects CSI where salt pools at 3000 ppm have around 0.2 units lower CSI all else equal.
Our current Recommended Levels are still inconsistent and low in CSI for SWG pools where they should have an even higher CH for plaster because 1) the higher salt level has the CSI be 0.2 units lower and 2) the higher CYA recommendation has the CSI be 0.1 unit lower and 3) the lower TA has the CSI be 0.07 units lower while the higher pH target only has the CSI be 0.05 units higher.
Basically, at the low end of the Recommended Levels (except using the highest recommended CYA) for SWG pools, the CSI is -0.56 which is quite low. For non-SWG pools the low end of the Recommended Levels is only -0.23.
I first brought up this issue 3 years ago though then it was even worse because the ranges were even worse (the pH low end was changed later on from 7.5 to 7.6), but my recommendation of changing the CH range to 350-450 has still not been done. I brought this up over and over again and yet the Recommended Levels have not been changed:
Re: What am I missing? Everything seems good except CSI 1/19/12
Is the Saturation Index always Reliable? 1/19/12
Saltwater Chlorine Generator (SWG) Recommended Levels 3/9/12
Re: How important is it to familiarize w/ CSI? 8/13/13
Re: Help with Pool Math 5/18/14
Re: CSI 11/28/14
Re: What does and what doesn't get discussed on this forum 4/18/15
Re: Micromanaging pool chemistry 5/20/15