Should this be a comment or answer? It's too long for a comment, so:
Many people try to fight against there being rules in music, saying that there are no rules and if it sounds good, it's good. In my opinion, this war against the existence of rules seems a bit futile, because what people mean by "rule" is just any kind of internal logic. Saying that there are no rules is a bit like saying that there is no logic.
It might work better to "embrace" talking about rules, and guide the discussion towards talking about what kinds of rules there are and areas of applicability of rules.
Quote from http://openmusictheory.com/tendency.html
In explaining musical styles, Leonard Meyer divides musical
characteristics into three categories: laws, rules, and strategies.
Laws are characteristics of music that are based on human biology and
psychology, and as a result laws are more-or-less universal. Rules are
culturally conditioned. They are hallmarks of a particular style that
are more-or-less universal within the style, but differ from style to
style and culture to culture.
In a musical style, there might be a rule or convention or tendency to use certain patterns, and adhering to the rules of that style is sometimes needed to make music that is identified as belonging to that style. Sometimes the rule is just limited to a certain period of a certain composer's work, or to even just one piece.
The problem with talking about rules like this is that there's no rule book. But if we remember to include the scope of a rule, then maybe it would ease the path to understanding the internal logic of compositions and arrangements. For example - you can set it as a rule to always stay in key, but a lot of musicians like to break that rule, because it can make the music more interesting.
What comes to the "Non Diatonic Harmonization?" question, why not provide some potential rules that could be used? Like Tim implies, rules are tools. You need tools to make music. Just don't say that the rules are universal, it's "choose your own rules". Choose what rules you want for your style.