I've read a lot of negative comments about Miracle Gro potting mix, but I can't say how true they might be. I've never used it myself. I prefer the Sun products. The Sun mix that's easily available in stores where I live ... even grocery stores like Albertson's ... is called Nature's. It has always worked very well for me. I buy it in the large bags for use in my outdoor containers, and small bags for my indoor plants. I buy their specialty mixes, as well ... African violet mix and Cactus/Succulent mix when I need them.
Sometimes I'll toss in some perlite into the mix ... I prefer it over vermiculite ... to loosen the soil a bit, but as a rule, I don't have a problem with the Nature's brand.
I don't actually tamp the soil down much in containers. I fill the container, tap it on the ground or countertop to settle the soil, plant my plant, then water thoroughly until water runs freely out the bottom. I might even water the plant two or three times in succession right after I've planted it. The purpose is to move the soil around and get rid of any little air pockets.
Whatever soil you use, make sure it's labeled specifically for use in containers. It should say "Potting soil", "Potting mix", or "Container soil" or "Container mix". Sometimes the "mixes" are soilless, but with the Nature's brand, I've found that true of only the African violet mix.