Robin, we appear to be talking at cross purposes here.

I really don't understand what you are driving at when you say

Major scales contain only Major triads - Minor scales contain only Minor triads.

Triads are formed by taking any given note in the scale as a root note, then adding the third and the fifth note following the root note in the scale. Thus, in the scale of C major, the notes of the scale are

C D E F G A B C D E F G A B C

the first triad (I) is formed by

CD E F G A B C (CEG) = major triad, chord of C major

the second triad (II) is formed by

C D E F G A B C (DFA)= minor triad, chord of D minor

the third triad (III) is formed by

C D E F G A B C (EGB)= minor triad , chord of E minor

the fourth triad (IV) is formed by

C D E F G A B C (FAC) = major triad, chord of F major

the fifth triad (V) is formed by

C D E F G A B C D (GBD) = major triad, chord of G major

the sixth triad (VI) is formed by

C D E F G A B C D E (ACE) = minor triad, chord of A minor

the seventh triad (VII) would be formed by

C D E F G A B C D E F (BDF) = diminished triad, and as I stated above would never be seen written on a Nashville chart. Either the arranger would require a V7 (GDDF) or else a diminished chord in which case he would, I imagine write IIdim (orIVdim, or VIIdim they are all the same chord)   

So a major scale yields major and minor triads, and a diminished triad, as of course do minor scales.