Jim--yes, you can start and A scale on the middle finger--that's the first moveable scale I showed--moved up one fret--this is a different position--starting on the pinky--

It is fair to ask, why play the scale starting with the pinky-- well, it offers a lot more options--

A)you can move from playing a scale to playing a chord without changing position--
B)You can move from an A to D scale (and chord) without changing position-
C)you can harmonize your melody or lead

The main reason for using different scale fingerings though, is that, some melodies are easier in one fingering than in another--so if your fingers get tied up in one position, you try it in the others til you find the place where it flows the most easily--this, incidentally, is what makes sight reading on a guitar is such a pain--