Chloramine-T is by far the most effective sterilizing agent, because it acts two-fold. It goes to far too describe the chemical particulars, but Chloramine-T's modes of action are both "traditional chlorine" and a specific antibiotic action that has nothing to do with Chlorine but can be compared to Sulfa antibiotics. That's why Chloramine-T is still used to wash out wounds in hospitals.
However - quite a few people are allergic to chlorine compounds, and the higher the concentration, the more likely such people will experience itching or rashes. Since Chloramine is not dangerous, you could just try a 2% solution, if you don't object to the chlorine smell.
If you keep the closed bottle in the fridge, it should keep for some weeks or months, as the decay of Chloramine-T in the dark and at low temp. is very slow.
However, it is always best to make a new batch every month or so, which is easy to do, as it's just a matter of mixing the powder into a small bottle of lukewarm water and shaking it.
I'd not mix the Chloramine-T with salicyl powder, I think you meant mixing it with vit. C?
The Chloramine-T should be applied only for a short time. I think it would be best to wipe it off, before rolling, with some demineralized water (the water you put into a steam iron or a refilable car battery - for sale in the pharmacy).
This to avoid that remnants of the Chloramine-T end up inside the micro-holes in the skin. Dr. Des Fernandez prefers not to have any chemicals enter the freshly microneedled skin, because it is not known if they interfere with the inflammatory process needed for optimal collagen formation.
Therefore he prefers not to use Betadine, because it is hard to remove it fully prior to rolling. We know that alcohol works, since there are many positive reports. Alcohol evaporates prior to rolling. Chloramine-T does not. So if you use it, the best way would be to wipe it off with demineralized water (that's sterile due to its manufacturing process). If you don't, it is unknown whether it negatively interferes with the collagen regeneration process. I think it's probably OK, but it is impossible to be sure (nobody really knows).