A sure way of wasting time is to choose a starting finger or believe it has any bearing in the real benifits of alternating fingers.
The art of alternating fingers (of any number) is that they alternate from any or all giving situations.
Let me explain some simple thoughts.
The plucking fingers when used correct are level, there are not "Longer fingers", in fact the tips are very close to being level. This is because the finger joints and knuckles are being used correctly. To see this, look at the palm of your hand with the fingers straight, they are as we know different lenghts. Start to bend the in slowly to the palm, at the point they become level is the ideal shape for that plucking hand, so no finger is any longer than the next. Look to the knuckles and joints in this shape and you will see they are not level because they are keeping the tips level.
Next thing to consider is lets say you practice 8 note scales in a cycle of fourths and fifths. So you will start on the E-string and play D major up and down the play G major up and down on the A string a fourth above. Then back a fifth to the E string and play C the same and the up a fourth and play F the same on the A string etc etc all the way down the neck till it can progress no further.
So that is two major scales, one on each of the E and A string from the same hand position. So the D will start on the index, then alternating means that the G must start on the forefinger, or vica verca. The fingers because of alternation and the fact that fifteen notes are being used in playing up and down the scale means that each new scale will have different starting finger.
So in the real sense of playing there is no starting finger as such, it is just reletive to the situation, and of course as a rule the correct amount of bend/flex in the fingers will give the hand the best shape to deal with plucking.
