On Musical Triads

I'm teaching myself guitar. Today, reading "The New Complete Guitarist," I got to the part on scales and chord theory. I already know how to play basic open and barre chords, but I feel I need to learn the details before I can advance my skills. Not unlike how I learn how to program. I very much expect this to change as I learn more, but here is what I am starting with.

1st: Major scales have a "root" note. C is the typical main scale (due to characteristics to be described later) and so its root is of course: C.

2nd: Intervals on the major scale occur between the notes. This gives us either a "tone" or a "major second" such as C to D or a "semitone" or a "minor second" such as E to F. The concept of a "second" refers to the ordinal position relative to the base note being references (C and E in the above examples).

3rd: Interval jumps (second, third, fourth, etc) on the C major scale are organized thusly:

• A single tone jump is a "major second"
• A single semitone jump is a "minor second"
• A double (three notes total, including the base note) jump that is entirely made of tone is a "major third." If there is a semitone in it, it is called a "minor third."
• With four tones or semitones, it is called a "perfect fourth."
• With five tones or semitones, it is called a "perfect fifth."
• For six and seven tones or semitones, it is called a "major sixth" and "major seventh" respectively.

4th: Everything listed above gets mucked up when discussing a non-C scale. Reviewing this as a simple letter arrangement (where spaces represent tone intervals):

• A: A BC D EF G A
• B: BC D EF G A B
• C: C D EF G A B
• D: D EF G A BC D
• E: EF G A BC D E
• F: F G A BC D EF
• G: G A BC D EF G
  

It appears to me that the 6th and 7th intervals become "minor" if there are two semitones in it, whereas the C scale only has one semitone in it until you reach the octave. On the B scale, it gets strange because the fifth interval (B to F) has two semitones in it, where as both the C and A scales have only one. This appears to make it a "diminished fifth."

5th: The concept of the "dominant seventh" describes the addition of the seventh interval, though I am not sure why. "Dominant" refers to the fifth chord on the major scale.