6) The spanish guitar has a strong urge to be composed in E phrygian.
That simply because the standard tuning is all E phrygian. For other tonalities you may use a capotasto, or simply compose in a manner where other tonalities make sense without alternative tunings and capotasto
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Well......, as a guitar player I will reaveal now that there are quite some beautiful other keys available. Usually one recognizes the composer who doesn't know all that much about the guitar when he writes in E ... [;)]
D is a wonderful key, as well as A. G is fine as well as are actually all the other keys.
I never had to use a capodaster.