There are many schools of finger style guitar. Each is a bit different from the other. Just to discuss my background and experience in this matter:
I started playing guitar when I was 10 years old, December 1952. I became familiar with Guy Mitchel, from his television program. He used his thumb a great deal.
I got involved with classical guitar and had instructors from the classical world including Fred Stockton, Rey de La Torre, and Andres Segovia. In the flamenco world some of my instructors were Carlos Montoya, Sabicas, Mariano Cordoba, and Juan Serrano. My interest in guitar included Blues, and country style, and I had some good friends who helped me out with those styles.
Merle Travis, Chet Atkins, Jerry Reed, and Tommy Emmanuel have something similar and yet different guitar styles. I use a bit from each of them. I had a Masters Class from Tommy, and he is a great teacher and person.
What I found was that there were two basic schools of thought:
“Do it my way or the hiway”, and
“do what works for you”. When you get to the point where you become your own teacher, you will know what is best for you.
With many instructors, and publications to choose from, I found what is best for me depends upon what piece of music I am playing. So what I have done for many years is to create my own arrangement of whatever piece of music I want to play. Before we had computers, I used pencil and paper. I still have arrangements I made more than 60 years ago.
Pulgar, Indice, Medio, Anular, and Menique are the names of the fingers in Spanish, which is how I learned to play guitar. It is abbreviated P, I, M, A, m. My arrangements indicate which fingers to use. You can create your own arrangements, based upon what works for you. Use whatever resources are available, and then select what you like.
One thing I do frequently, is to use I, M on a single string, when speed is necessary. The concept of I on the third string, M, on the second string, and A on the first string, is sometimes necessary, but not always. Again, it is up to you do understand what works best for you for any specific piece of music.
Within a specific piece of music, you may well change the rules, as required to obtain the desired result.
I used the Gibson
“Learn and Master” series, as well as
"Griff's ABGU", Joseph Alexander's
"Fingerstyle Blues Guitar" and many others. Each has it's good points, but I must say that I enjoyed Griff's approach the most. It is up to you to find
"what is best"! Pay your dues and make that determination. If you like the music literature you are using then stick with it and do not bother discovering what other approaches there are. Have fun!
Tom