If there are rules, I doubt I know them anymore, nor could I teach them to anyone.
Horticultural rules very with the climate, IMHO: for the most part I learned them intuitively by spending time with each tree when watering or just observing keenly as the seasons change.
Artistic rules are just a matter of having the eye, and having hands imbued with non-verbal skills to create what the eye wants. Once again, I found over the years that I would just hang out with my trees for a while when I watered them, and occasionally they would sort of "speak" to me one day and show me where I had to go with them.
Of course, this reminds me of a poem,

a great one by Theodore Roethke, and his most famous:
The Waking
I wake to sleep, and take my waking slow.
I feel my fate in what I cannot fear.
I learn by going where I have to go.
We think by feeling. What is there to know?
I hear my being dance from ear to ear.
I wake to sleep, and take my waking slow.
Of those so close beside me, which are you?
God bless the Ground! I shall walk softly there,
And learn by going where I have to go.
Light takes the Tree; but who can tell us how?
The lowly worm climbs up a winding stair;
I wake to sleep, and take my waking slow.
Great Nature has another thing to do
To you and me; so take the lively air,
And, lovely, learn by going where to go.
This shaking keeps me steady. I should know.
What falls away is always. And is near.
I wake to sleep, and take my waking slow.
I learn by going where I have to go.