If you're gonna finish in any container smaller than 7 gallons, you won't be able to use super soil as a just-add-water recipe. You're going to have to give liquid feed, and if you're going to that trouble, why even use super soil anyway? The whole point of using super soil is for simplicity. In my experience, even 7 gallon pots are too small. 10's are the way to go. I use CO2 and 1kw's though and that's probably why the girls eat so much.
Also, it wouldn't matter what shape my pots are because my girls are MUCH wider than the pot when they finish. Like, two to three feet wide. I can barely squeeze four plants under a 1kw. I root cuts in an aero cloner, toss 'em in #1 pots (which aren't really a full gallon, btw) full of Roots, top them early, veg them for about three weeks, transplant into 10 gal pots with 4.5 gallons of super soil on the bottom, plain Roots on top and then veg 'em out and LST to about 30" tall with between four to eight tops depending on the cut. Plants finish between 4 and 5 ft tall, big fat bushes.
Good luck to the OP. I think you may be over thinking things. Follow sub's instructions EXACTLY and you'll do fine. His method is very well documented on numerous forums, including this one, and on YouTube.