Wait a second, I never said you should flush... I was assuming that you already had done a minor flush to correct your pH problem. If you're using fresh soil and haven't yet added any nutrients, there's probably no need to flush. Especially if there's no perlite and your soil holds moisture well. In cases like these, flushing could actually do more harm to the plant than good. Flushing is appropriate when you need to clear excess salts from the fertilizers out of the soil, when you've over fertilized by accident (emergency flush), and when you're a couple weeks away from harvest and want to rid the pots of excess fertilizer in the soil. Definitely don't do it just to do it.

Since some of your soil already has nutrients added, you'll only want to fertilize those with about a 1/4 strength solution to begin with. You can raise the amount you use as you see how well your plants are tolerating it. As for the Superthrive, don't be too liberal with that stuff. I think one drop per gallon of water is the general recommendation for it.