Ecke,
I would like to water my pots (preplant) with 200ppm 20-10-20 or even 300 ppm.
And then water in (post plant) with a fungicide drench.
My thoughts are I will be feeding and protecting them right from the start.

Is this okay?

Your thought is good, but it is not necessary to fertilize before planting. What may happen is that when you apply the fungicide you will leach the fertilizer thus wasting it. So we suggest you to fertilize at transplant and apply a fungicide within a week.