Captain Birdseye wrote;
what I try to avoid, when im using harmony lines are parallel thirds,

A few years ago, I took Associated Board Grade 6 Theory and there we were taught that parallel fifths and octaves were the big no no. Parallel thirds were OK and, in fact, are widely used.

Obviously it is not essential to stick to these rules, except when taking an AB exam <g>, but knowing them means that you are more likely to break them consciously for effect.

You are right about the intervals, though. The thirds, major and minor are more "out" in equal temperament than in most historical temperaments. The sixths, incidentally are equally out.

A few years ago, I did a comparison of the notes of the harmonic series with equal temperament, so I could make tuning adjustments to MIDI files. I reproduce the data below.

                     ET    harmonic series;
                    cents  ratio  cents
  semitone          100    16/15  111.73
  major second      200     9/8   203.91
  minor third       300     6/5   315.64
  major third       400     5/4   386.31
  perfect fourth    500     4/3   498.04
  augmented fourth  600     7/5   582.51
  perfect fifth     700     3/2   701.96
  minor sixth       800     8/5   813.69
  major sixth       900     5/3   884.36
  minor seventh    1000     7/4   968.83
  major seventh    1100    15/8  1088.27
  octave           1200     2/1  1200.00