Re
Most people here feed ft throughout the entire life of the snake. Once you've gotten him over the hurdle to eating ft foods, why would you want to switch back?
Frozen foods are easier to store, never have to be fed, never have to suffer, and most importantly don't have the chance to injure your snake due to bites or parasites.
Once he's eating steadily on frozen/thawed I doubt he'll think twice about eating it continually. He may go through periods of his life where live would be more tempting to eat, such as breeding, etc. But if he's on pinkies right now, I'd say he's a few years away from that.
One month isn't a good indicator of growth. Bearded dragons and snakes grow at different rates, why? Because they're entirely different species. Beardeds have a higher metabolism than snakes and are 10x more active. Snakes take everything rather slowly, and that includes digestion and ultimately growth.
A 20 gallon long tank sounds fine for the entire life of your snake. I doubt you'll ever have to move him to another enclosure.
I think you need to invest in the Corn Snake Manual, can be bought at Petsmart, Petco, and many other pet stores. You can even find it at online bookstores. Most of the questions you've been asking can be easily found in a book.