It really isn't as complicated as your making it, your forgetting your growing a weed. Weed is an extremely hardy plant, it grows vigorously as long as something doesn't eat it, or kill it, or someone doesn't water it too much or feed it too much. You soil will work just fine, but add perlite too. Careful with the stuff that has food already in it and says it feeds for 3 months because when you get to flower stage you don't want to feed your plant nitrogen. So, just add some perlite and you should be good to go.