I'm not sure how to tackle your question. So, I'll start by throwing my experience at you, and my opinion as well. Hopefully someone can join in, and shed more light on the subject.
The times that I've installed "CT" cans, they were 30" X 42" because that is what the PoCo provided. They have rules on "where" the conduits can penetrate the can because they have to install their mounting hardware for the "CT's". So, "where" and "how many" conduits go in a "CT" can is up to your PoCo. You are at their mercy, unless the "CT's" are being used for something other than utility metering. A "CT" can is a cabinet, which you correctly pointed out in your post. No doubt about it Article 312 is where you should look.
The other box should be considered an Article 314.28 pull or junction box IMHO. The "off the cuff" rule for this is "V6, straight 8". Just the opposite of our normal thoughts about cars.
Eandrew, if you can give some more information, it will help. For now, my wife, The Watt Mama, is pestering me to get off the computer. I will post more later.
Get your motor runnin',
The Watt Doctor