I think you're missing the big picture with organics and bacteria issues. The reason organics and hydroponics are messy to mix is because the sugars in organic ferts typically act as a food source for the bad bacteria, leading to more outbreaks. Having organic substances present to the plant via foliar feeding, unless it drips down into your res, won't provide food to anything but the plant itself, so its safe from causing bacteria outbreaks.
That said, I don't see any reason to foliar feed a plant that young. What exactly are you trying to accomplish by incorporating the product, aside from just using it because you have it?
Personally, I only use a foliar spray with kelp extract, and that's because the caspian strip makes it near impossible for the plant to utilize it through the roots. Same goes for any sugar product, though I don't use any. So yes, I would recommend you use that as a foliar feed if you're set on using that specifically, but not for another 2-3 weeks or so. The hormones the plant uses from the kelp are relatively useless before that as they increase overall stem elongation and not exclusively root production.
Not too sure specifically about root 66 compared to similar products, as its ingredients are kelp, sugars, and trace elements including boric acid(which has been shown to be toxic to plants in high enough concentrations). IMO seems like a redundant and relatively wasteful product aside from the kelp extract in it, and based on how much is in it, likely not a very good source of it if you're set on using some type of kelp.
