@redpandamaniacal From what I can tell, the pinkies were somehow linked to the rings on them, so it's less important whether or not the pinkies are connected and more if the rings are there. My guess is that the rings themselves were used to grant him power and subsequently changed his fingers to represent the power that was given to him. She cut off his pinkies just below the rings which caused them to fall off, so that is the cut off point(pun intended). As for why he wouldn't go for the rings instead in that instance, it could be that he misunderstood and saw the pinkies themselves as the power, or it could be that the rings have restrictions where they can only be used on the pinkies or just simply only work once and no longer grant power to others after they have done so. Could also be that the rings are inconsequential and were just there because he wanted some bling on his
dicks pride.
I realized after I finished the previous paragraph that I didn't properly give you a potential answer to your question, so here goes: A pinkie is the shortest digit of a hand(not including the thumb that
appears to be shorter), however the hand is the entire bit of flesh from the wrist to the tips of each finger. While one could indeed argue that cutting off a hand means that
technically all subsequent fingers are considered cut off(from the body), because the fingers are still actually attached to the hand they are therefore not cut off and instead the hand is refereed to as it's own entity("The hand was cut of," instead of "the hand and fingers were cut off"). The same can be said for the head, but not in the way you think. When something becomes two parts of a previous whole, for instance a person becoming a head and a body, it is important to note what was removed from what. In the case of a decapitation, the head is removed
from the body, and thus we say "his head was cut off" rather than saying "his body was cut off." With this in mind, if Ryuu had cut off our demon aide's head instead, then technically the pinkies wouldn't be considered cut off because they are still connected to the body through the hands, arms, and shoulders. As such, while you can technically consider fingers being cut off when a hand is cut off, the same can't be said for the head since the body itself, or rather the torso, is considered the base from which every other part of the body is removed
from.
I apologize if this is confusing. I tend to ramble a bit, and when I'm given the task to explain something I often do so in varying degrees of convolution. That is, what I say doesn't always make sense to others even though it is accurate. Assuming, of course, it is indeed accurate and I didn't make a mistake in my logic somewhere. At that point, however, the challenge becomes finding out where the mistake is in the convoluted mess that I have presented.
With all that said, I hope I was able to answer your question.