To answer that question, I need to know your pH and CYA levels. That way I can calculate the actual carbonate alkalinity. That's the main pH buffer that prevents a "crash" of pH downward from acid sources.
Your thinking is correct that the main risk you have at the low TA level is that you don't have much pH buffering against acidic sources. That's why I want you to tell me your pH and CYA so I can tell you exactly how much acid or Trichlor or Dichlor or MPS you would have to add before a "crash" in pH. It will give you some sense for the risk level.
Also note that you can minimize the amount of acid you need to add by not only targeting the lower TA, but also a higher pH, say around 7.7 or 7.8. Don't try and lower the pH below 7.5. That will help reduce the rate of carbon dioxide outgassing. The amount of over-carbonation of the water at various pH and TA levels is given in this chart.
The use of 50 ppm Borates would be another optional item to provide more pH buffering, but primarily against a rise in pH since the borates buffer gets stronger at higher pH. It won't help against a pH "crash" going down in pH (i.e. against acids) since the borates buffer gets weaker as the pH drops. You can see the relative buffer strengths at various pH for the carbonate, CYA and borates buffers in the thread
pH Buffer Capacity.
I know this would be much easier if I gave you all results from a Taylor or TF-100 kit, but it'll be at least four or five days before it arrives. If you want to tackle this question based on two, three-hour-apart test results from a pool store that I did yesterday after reading your request, go ahead. I've got it below. If not, we can wait until later when I have a complete test kit. I'm not sure if these results are an unusual amount of variance or not for the CYA, but that's what I got on the reports. All the other levels seem pretty consistent to me. I told the pool store folks what I was doing and what was going on. They're cool with it.
FC 4.7 4.2 (My OTO test; dark yellow OTO 5.0ish at or about 4 pm--immediately prior to gathering the first sample taken to PS).
CC 0 0
PH 7.4 7.5 (My tester showed 7.6 immediately prior to gathering the first sample taken to the PS).
TA 43 42 (My acid drop test turns clear after 4 drops; indicating 40ish).
CH 124 121
CYA 40 46 (My tester doesn't test CYA).
I know you stated in an earlier post not to add acid until PH rose above 7.8. That's exactly what I've been doing for many months according to my Swimline PH tester, but now suddenly I've sort of ended up with it staying stable at around 7.6 (according to my test), and 7.4/7.5 (according to the pool store test). Please understand though, I wasn't trying to get PH down to 7.5 and hold it there; I assumed it would come right back up like it has for the last year-and-a-half and all I've been trying to do is keep it in range. I understand what you've written about letting the PH stay as high as 7.8; it's just that the mid 7's is where it's ended up for now, and I've not done anything but daily replenishment of chlorine via bleach since that last adjustment.
I didn't log it, but I think it's been about 6 or 7 days since I made this last PH adjustment. At the time of the last adjustment, the color test was brighter than the 7.8 (sort of in between what the chart shows for 7.8 and 8.0). This is sort of how it's been normally been going for the last year-and-a-half and about the same way I've been adjusting it for the last few months. The calculator shows 6.9 ounces of 31% MA for my pool to move it from 7.9 to 7.5. Whenever it has shown this same brightness of red, I've been adding 7 ounces of MA. Occasionally, I might do the evening test and it'd be slightly brighter than this 7.9 reading, but the day before is was not as bright as 7.8. In those cases, I've been adding 8 oz's. But I've not added more than 8 oz's on any one day since Spring. It just seems like (not for sure, because I didn't document it) that the larger PH drops created more work for me, because the TA would drop out quicker. Using this 7 oz's per adjustment method, I've been testing TA about once per week and adjusting TA about once every two weeks. I used to adjust TA up to about 100 and would take a couple of iterations to get it there. And then I learned that was not the TFP recommendation. I then shot for 80. This probably slowed the PH creep somewhat, but not enough for me to notice it. I guess also that TA 80 could have lowered the natural high PH level for my pool, but I would not have known that since I've always adjusted it once it went above 7.8.
I have never done a PH test result showing yellow, but last year when I'd try letting the PH climb to a very bright red, I might have added as much as 16 oz's. I was doing this to try and do less work, but then it seemed like, at least every time I took notice of it, that every time I did a big PH drop adjustment, it would lose alot of TA. And in those cases, I might get a PH reading as low as 7.2 on my color chart.
