Just use regular potting soil. No need to mix it with anything. Any high quality potting soil will do just fine. Whether it's miracle or something else. Don't get the cheapest potting soil you can find either. From my experience, my peppers have never really done well in it. You'll need to figure out which nutrient solution you're gonna give them too. After about a month, the nutrients in the potting soil are pretty much depleted so a weekly dose of a dilute nutrient solution will be needed. MG granules are fine if you're looking to just keep it simple but there are a lot of other options out there.