As noted above, your soil mix will feed them for at least a couple weeks. After that, I would keep your fertigations low initially, perhaps half of the recommended schedule, and ramp up gradually only after you see color lighter than you would like on the lower fans. The nutes are fine, the problem, and it is normal for soil mixes, is that it is difficult or impossible to know exactly what is still in the medium when you start with the nutes, and that can cause imbalance issues. IMO about all you can do is start light, watch your plants, and cross your fingers.
As for the pH perfect thing, it did not work for me the first time I used it in my solo grow last winter. I had to pH, and the buffer in the mix actually seemed to fight the process of getting the pH where I wanted it. I tried the stuff on my solo run just to see how it would work, but will not be using it again. It is too pricey given that for me at least it does not eliminate the need to adjust pH, and may make it more difficult.
Good luck with it.