The right way of alternate plucking

[Garun]

For slower tempos, as you said, index finger would be ok, so you get even tone.
For faster lines, I couldn't play anything fast without alternating, strict alternating that is.
And about length of fingers, that doesn't matter. My middle finger is 2cm longer than index, but I play it more like classical guitar (Gwizdala and Willis have this aproach) and I don't have any problem. It is easier for me to play, and octaves are falling natural to my ring finger.
This is my approach, it isn't the only one, but it worked for me about a year now without any problem.

 

[hintz]

Another option preferred by some players, especially for fast tempos, is to use three plucking fingers so each triplet starts with the same finger.

This/\!! Learn the 3 finger technique for these rhythm's, with this technique its a breeze!! Another option is to use the chuck Rainey technique of using 2 fingers *but* double hitting with index on a upstroke, so it would be: M, I(down),I(up), etc... You can also combine both for straight 16ths :R,M, I(D),I(up) and play some Sheehan style badasssery!!

Also, practicing good alternating fingers is great in the woodshed, but in real situations you will find that strict alteration isn't the best but instead raking whenever possible and going with what *will* eventually become 2nd nature! So by all means work on good strict technique, but remember what Wooten say's"If your thinking, your stinking!", and that's what its all about, letting the music flow from you with as little effort or thought as possible!

Good luck!