My favorite potting soil is Fertilome, or Jungle Growth (at Lowe's). As long as you didn't buy the Miracle Grow Seed Starting Mix, you'll be good. I try to find potting soil without fertilizer & add a handful of lime & Tomato Tone fertilizer in each container-- but everybody does it different I'm sure. :)

I use MG moisture control potting mix mainly. You should be fine with the regular MG mix. Maybe add some kind of moisture control additive like peat.

I grow about 50-60% of my plants (about 65-75 tomatoes and 40-45 peppers) in 4 gallon or 5 gallon containers with drip fertigation. I use ProMix BX and municipal compost from a nearby town. If you are manually watering & fertilizing, I'd add about 20% composted or dehydrated manure and 20-25% compost. I use as much as 40-50% compost.

The MiracleGro potting mix is fine if it's the MIX and NOT potting SOIL or GARDEN SOIL. You can eliminate some of the manure if the M-G has added fertilizer. I also usually add time release M-G or Osmocote or a generic time release like Expert's (Walmart brand) but read the label and make sure it's the vegetable formula & all the N-P-K #s are the same (14-14-14, 10-10-10, 17-17-17, etc, not the 24-9-16 or whatever #s) or the middle number is the higher of the 3. For organic, Neptune's Harvest is the best fish fertilizer, at 2-4-1.

With containers, you will generally need to water daily or 2x/day on smaller size containers. Regular watering and fertilizing are more important than pot size. I've grown some indeterminate cherry tomatoes in containers as small as 6 quarts, and some 2lb oxhearts in 4 gallon buckets, all with inexpensive drip irrigation (you can find basic drip irrigation kits as low as $9 thru timer systems at $40.) The bigger the container, the more water at one time, but the less frequency of watering & fertilizing needed.

Hope this helps.

Mark