I use westlands seedling soil and all purpose soil to which i add lime, perlite, vermiculite and one third bio canna terra organic soil. I hate all soils for one reason or another but organic soils seem to be the best nutrient wise, biocanna organic runs at 1200ppm at most so can be a little too hot for small plants. Most of my soils have a good dose of nitrogen to start with, i have no problem growing with organic nutes no matter what soil i have used.
I know little of pro mix, is that the biobizz range or another? I find almost all hydro shop guys know sweet FA about growing, ask him to show you his grow first before you believe him. Peace