Hi Connor,
Speaking from a Hosozao point of view, changing bridges changes SO much. Using a zouge koma makes a louder sharper tone, whereas bone koma make a much more mellow sound. I only use 3.5 height. 3.6 is a REALLY high koma, and isn’t always available everywhere. 3.5 is normally the height used by the lead shamisen player because it is really loud. In my opinion, higher bridges amplify more when it comes to Nagauta. The intonation might be a bit funky after switching because the tension of the strings change. Even still, it might just require a quick fine tuning if anything at all.
Sometimes just for fun, I’ll switch to a Jiuta koma (water buffalo horn). The sound is totally different. Its a really mellow, deep sound.
Kouta koma (Redwood with a zouge lining where the strings touch) are really nice in the high register. If you are playing with a bachi instead of fingers, they make a really sharp clear sound up there. They are higher, and because of that are a bit more difficult to do really fast sukui (upstrokes) on.
Bekkou koma is not really used with hosozao shamisen, and Kevin or Kyle would be able to talk about that with so much more detail than I can. It is more of a Tsugaru/Really expensive Jiuta koma material, but is definitely useable with hosozao if you wanted to.
1-4 is completely impossible for most (unless you have monstrous hands), even if you use pinky. The farthest most will get is a 3# (which isn’t really a note…), and that is super uncomfortable on the index finger.
Not sure about Tsugaru or modern compositions, but for the rare bar chords in nagauta, the middle string is always left open.
An example of this is in the piece “Tsuna Yakata” or “Tsuna Ken” meaning “The house of Tsuna”. Here is Martha Rinoie playing this piece, and you can see this technique around 1’48".
San no ito : --2-2—2- (Index finger)
Ni no ito : --0-0—0-
Ichi no ito : --2-2-2-2- (Middle Finger)
Hope this helped a bit!
Joe