i have a slightly acidic pH to begin with, between 6.5-7.0 depending on how aged the water is.
If it's 7.0 out of the tap, then 6.5 is your actual source water pH, or at least where it's at once it's in your tank and exposed to it's acids (not sure if you're giving us 6.5ph 'aged tap' or 'in tank'). Either way, the 7.0 straight from the tap would be a temporary increase from the treatment plant additives (unless it's a well) which usually precipitate within a day or two.
Try not to concentrate on pH, rather the KH after you have the SMS in the tank, because it's own pH may have an effect on the tank's pH that isn't necessarily related to it stripping carbonates out of the water. Your goal is stability, so it's the KH your concerned with over pH. You also have the wood tannins which are effecting pH, the plants and oscar will have no issues there, nor will they care about the KH. Yes, no buffering capacity can leave a larger window for pH changes, but you have to take into consideration what is causing the fluctuation and decide if it's detrimental to your fish, for example, is it only an increase in O2 or CO2? Is it only a temporary increase from non-aged tap water on water change day, caused by treatment plant additives that the fish have no issues with either? All situations that are generally not dangerous to fish unless the cause is a CO2 injection overdose.
If it's any consolation, there are plenty of folks here running pressurized CO2 with no KH and see no bad fluctuations in their pH. If wood is your only main source of acids then you only need to worry about a pH shift if you fail to do regular water changes. Otherwise, the changes will replenish carbonates while removing acids, keeping things stable, though you may have add a bit of baking soda for a short while after adding the SMS to keep KH where it usually is, considering a few folks have reported a drop on KH from SMS. I wouldn't doubt it's happening due to SMS's high CEC, bt I personally had no problem w/ my SMS.