Thanks for your reply. The Klein's butterfly is listed as both peaceful and easy on Liveaquaria and every other website I have checked, and is recommended as a possible beginner fish do to its hardiness. The pearlscale butterfly, which is less hardy than the Klein's (listed as moderate/intermediate) also grows 6" yet it is recommended for a 70+ on Liveaquaria. The Burgess' butterfly grows 5 1/2 inches (which is fairly close to 6") and is recommended for 70+ on Liveaquaria. The Klein's butterfly is neither aggressive nor difficult to care for, and is no bigger than the pearlscale and close in size to the Burgess', so those reasons are not necessarily why it is listed as 120 gallons+. All the fish listed as 120 minimum are listed as moderate-difficult to keep except for the Klein's.
I know that horizontal swimming space is just as important as volume (and technically a 120 gallon is still four feet long even if it is more than double the volume of a 55 gallon) but I find it weird that that Liveaquaria would recommend 70+ gallons for a 7in kole tang (yes I know that the size of a fish is not the only factor, but we are talking about a tang) that is also moderately difficult to care for, and is semi-aggressive, when I would imagine it would require more horizontal swimming space than the Klein's butterfly.
Whenever I look for stocking information I always check multiple sources and do my best to research the needs of the fish I plan to get. Most of the time I find fairly clear answers, even if there are a few outliers. For instance, a few people say they have kept a yellow tang in a 55 gallon with no problems, but because the vast majority of sources you research and people you ask say a 55 gallon is way too small for a tang, I would not even consider adding one to my tank. In contrast, in my research for the tank size for a Klein's butterfly, the only source I have yet to find that lists more than 50-60+ for a single Klein's is Liveaquaria (not that there aren't other sources somewhere that say otherwise). Normally I would see at least a few other sources that say something larger, whether it be 75+ or higher, but I have not found one. From my understanding a 55 gallon is suitable for a single Klein's butterfly, but as Liveaquaria says different, I am confused.