Rules narrowed down: Chord extensions

Below is a narrowed down version of the ruleset.  In this post we'll be looking at rules relating to chord extensions. Rules from previous iterations have been removed and edited.  A finalised version will be posted later.

These rules exist to "spice" up existing chords.  There's little reharmonisation involved in these chords.  Some tunes can be reharmonised using these rules alone.

RULES

1) If  chord is a simple triad, add the seventh relating to the key.  Eg. in key of C, C major can be come Cmaj7.  E minor can become Em7, G major can become G7, etc...

2) In most cases, simple triads can have the extended variations of their chords attached to them.  Eg. C major triad - add Maj 7, Maj 6, Maj 9, etc.

3) Major chords can be replaced with the relative minor chords: for example, Gmaj7 can become Em7, etc.

4) If the melody note does not fall on the fifth or perfect fourth, a flattened fifth can be added to a minor chord to create diminished colour.

TENSION NOTES - A list of notes that can be added sporadically to chords for colour:
ONLY ADD IF THE MELODY NOTE DOES NOT FALL ON A NOTE A SEMI-TONE APART FROM THE ALTERED NOTE

Major 6th - Add 9
Minor 6th - Add 9
Minor 7th - Add 9 or #11
Minor Major 7th - Add 9
Dominant 7th - add 9 OR b9, 11, #11, 13, b13
Minor 7th - add 9, 11
Minor 7th (lowered fifth) - 9, b13
Diminished 7th - Any note a whole step above a basic chord tone.