None of the rule of thumbs, or theories, or any of the old jargon are true. Maybe some are, but when it comes to the shotgun, they are almost always false. I'm only aware of one that is true "Less powder, more lead, shoots far, kills dead"
The square load is equal volumes powder and shot. I don't know the history of when the idea that a shot column of equal height to bore diameter came to be. It's a idea that needs to be in the trash. It had no use from past documents, and is sure doesn't belong in modern times. Modern testing, such as high speed cameras have shown shot string is not as simple as more shot = longer string. From the research I've done, best book I've read being Shotgunning by Bob Brister, it isn't worth worrying about shot string until you've met a whole bunch of other criteria first. Shot string is what it is, accept it, pattern your gun knowing it is there. The farther you are shooting, the more it matters. I doubt too many have a muzzleloader capable of really long shots where shot string is worth worrying about.
There's nothing special about a 16 gauge. It is what it is. In muzzleloader terms, a lot of people find it to be a great choice in a SXS, not being too large like a 12 or 10 gauge, especially in a flintlock, which is rather wide at the locks. In modern guns, certain guns really shine in 16 gauge. Others do not. But there is no black magic because you bored out the tube to .665". You can shoot more shot at less pressure than a 20 gauge. I'm not sure it will matter, as I'm guessing the Kibler gun will use the same OD barrel for both, making the 20 gauge barrel thicker, and presumably more pressure.
Really the biggest factor here is handling. I'm guessing this gun will have a ridiculously long barrel, like 44" or 46". I'll take the lightest barrel, and in this case that is the 16 gauge.
For the record, I'm a 16 gauge fan. I like my modern 16 gauge so much, I used it too shoot my bear this year.