Since you don’t mention whether the 15 ppm is free or total SO2, it’s tough for me to refer specifically to either the instructions or your protocol. However, I routinely add up to 30–40 ppm total sulfur dioxide to my reds pre-primary fermentation and then inoculate with my ML bacteria after the wine is dry (from a residual sugar point of view). Since free sulfur dioxide binds readily to sugars and aldehydes, by the time primary fermentation is over, your free SO2 is essentially zero. This makes for a relatively safe environment for your ML bacteria of choice, which are not affected by bound SO2 at that concentration. If you’re keeping your pre-fermentation sulfur additions to a reasonable level, there’s no reason to wait, as you suggest above, before you add your ML bacteria because post-primary the levels of free (biologically active and therefore available to interact with the ML bacteria) sulfur dioxide will have dropped to a safe (negligible) level.
You’re right to hint that leaving long periods of time at low levels of sulfur dioxide is risky for wine. You never want to leave your wine unprotected. A rule of thumb for me is that if it doesn’t have at least 25 ppm free SO2 a wine has to be going through either primary or secondary fermentation and therefore producing carbon dioxide gas for me to feel comfortable. This means that I get primary going quickly, after that’s dry inoculate with my ML bacteria and get that going, and then when that is determined “dry” I add enough total sulfur dioxide to maintain a free sulfur dioxide of 25–30 ppm. Check your free SO2s at least every two months, adding more with topping as needed, because free SO2 eventually gets bound up as total SO2 and loses its effectiveness. The lactic acid bacteria (LAB) for MLF is also sensitive to total SO2, so be sure to follow the manufacturer’s instructions carefully.
