I don't think mixing Safe-T-Sorb in with the soil will cause any problems. Although, I'm not sure there would be many benefits to it either. I imagine it may slightly reduce the amount of compaction of the soil layer, but I think that's pretty minimal to begin with. (I'm currently running a tank with ~2" MGOCPM, and a ~2" flourite black cap, and there hasn't been any noticeable compaction in the ~8 months or so since I set it up.
If you're never planning on having plants in the sand, then it might be easier to section that area off with flexible plastic (like cut from a 2L soda bottle, juice bottle, whatever), and use just sand there.
As to additives, I think it's frequently recommended to add a scattering/dusting of dolomite and muriate of potash to the bottom of the tank before adding the dirt. I believe the dolomite is to help stabilize pH, and provide some Ca and Mg. If you have any on hand (or even crushed oyster shell, crushed coral, marble chips, etc.), I don't think it would hurt to add in a handful or so. If you don't have any, I wouldn't stress about it.