Hello everyone, I'm in the D03 group and I have some experience with the "folk guitar" approach of using p for the 3 bass strings and i, m, a for g, b, and e respectively. This is how I learned folk guitar and it carried over into my classical playing for a while. There's nothing really wrong with this unless you're playing successive notes on the same string. As someone else pointed out, then you need to alternate right-hand fingers in order to play a quick pattern of notes.
I have a couple more thoughts about this:
I tend to use the "a" finger when the thumb is playing the low E string and the melody note is on the high E or B because it makes the two simultaneous notes easier to reach, than if you used the "i" or even the "m". But this is not a hard and fast rule, because it is possible to use the other fingers in this situation, just not quite as easy. Sometimes you will need to use "i" or "m" in a case like this because of the overall fingering for the passage, which requires you to use "a" on another string maybe. So there are a lot of factors to consider. An overall rule is to make the easiest, least awkward fingering that gives you the sound you want. (I sometimes find that I can get a certain sound from the nail of the "m" but not with the "a" - my "a" nail is a little differently shaped, but this may also be something that I need to overcome!)
When I started out I tended to use the "a" on the high E whenever I could - later I found that if the thumb was playing the G string then "i" or "m" might be better.
I will sometimes break the "alternate the fingers" rule,particularly if the tempo isn't that fast. Sometimes I want the same sound on each note and can get it easier by using the same finger again, sometimes it's just easier in a slow tempo. This might just be my idea, don't know if it's really approved. Of course you frequently use the thumb in a repeated way and this is okay if it's not too fast, so there is an argument for it.
Guitarras: 1973 Manouk Papazian (Spruce/Morado), two I have built, and an old Telesforo Julve parlor size