You won't be disappointed reusing that 'old' soil brother.
It's chockfull with all them bennies from the previous grow. If you have some enzyme product, like Pure Zyme, add it as well to the soil mix. This will help in digesting them small bits and pieces of root that was left in the old soil.
And like I said, if you reuse the old soil the Startrex isn't necessary. Using only half of the Silicium Flash can't hurt. It's a combo of a fast acting nutrient mix which will show results in mere days and slow release nutrient mix which will last for two months. And you can always top dress the soil with it if need be. Just make sure you work it into the first inch of soil before watering it in.