Thanks 
Crisscross, Kay, and Bertram!
crisscross wrote: "... Is the 1/4 size Yamaha guitar tuned in fifths like two tenor guitars built in one?"
Yeah that would be one way of looking at it. 
I made a typo 
in my video, it's a *half* size guitar, not quarter size. I went up to my earlier post just now, and made a note of that. I don't know how I got that mixed up. The manufacturer fractional designations are confusing anyway. This one has a body size similar to a baritone uke - I almost bought one of those instead but I wanted more than 4 strings.
But to answer your question:
The CGDAE tuning I'm using, theoretically allows for playing stuff designed for both CGDA *and* GDAE instruments, just by shifting the fretting hand sideways by one string, without having to re-tune anything...
I say "theoretically" because this particular instrument seems to only have decent tone on the bass C, G, and D strings... whereas the treble A and E strings just kind of go "plink".
Not sure why that is, maybe because I'm using a flatpick on nylon strings, or because it's an all-laminate inexpensive 'student' model of guitar and that might be as good as it gets. 
But I like the sound of the low C, G, and D strings, those are fun to mess around with. 
Bertram Henze wrote: "... The little guitar seems to be made for that."
Yeah I think I've finally found its niche, what it fits in best with - sort of a pseudo almost-bass thing with that low C.
Fortunately, it doesn't have the over-ringing too-much-sustain in the bass strings like my other fifths-tuned guitar (the solid-body electric) does. On the electric, I've always had to stuff foam rubber under the low bass string to stop it from perpetually ringing (sympathetic resonance). Whereas the little acoustic classical guitar I guess just isn't responsive enough for sympathetic resonance to be noticeable, or something.
I'm starting to remember now why I used to have so many instruments years ago, because they each were better for different situations, or - viewed as "glass half-empty vs half-full" - they were all deficient in one regard or another. 
