Hi Walter,

A few random thoughts in response to your question:

Some scales aren't designed, they evolve over time in a
musical culture. Later they may be described by theorists.

Musical instruments all have unique capabilities and
limitations in their production of sound. Aside from a
few indications unique for the violin, violin music is
notated with discrete notes. However if you look at a
violin melody with pitch detector software, you can see
it looks very little like a piano melody or even a flute
melody. So fixed scales are abstractions.

As for designed scales, the goals and methods of different
designers can be very different.

> 4) Why would a composer choose a specific tuning or scale
> over another.

Different scales are suitable for different music. If one
wants smooth chords, one will have an easier time with a
scale of JI or near-JI values, and the converse.
Also one can choose the number of tones in the scale, which
is related to the size of the smallest intervals available
throughout the scale.
Not much is really known beyond this, but there is no shortage
of theories. At this stage, every composer must experiment
and find out for his/her self. If you want some ideas, have
a look at the Scala scale archive or say something here about
the kind of music you like (or the kind you want to write)
and people can give you suggestions.

-Carl