Feel free to break All the rules of typography + calligraphy.
http://typophile.com/files/Jawi%20compare.pdf
As long as you keep the Arabic Arabic.
http://www.qalambartar.com/

It's ok as long as the word form retains a natural feel. There are structures in Naskh that can be brought into Kufi styles (like the middle Ayn) without creating problems. You need to maintain the same speed of movement and formality of structure though.

Need to maintain the same speed of movement?!

Thank you both.

I had to analyze the basics of five calligraphic styles, to understand (at least superficially) the nature of Arabic script. And finally chose Roqah as a guideline for the future font. It's characteristic compression of forms seemed to correspond with my idea of reaching some highly expressive, biomorfic movement - with the minimum amount of resources. At the stage of reconciling the demands of the style, formal idea and the "readability in the eyes of Pole", I started to be in doubt as to whether introducing some new issues, doesn't look eclectic (like, say, cursive Blackletter in Latin).

Nadine, I will think for some time about what you wrote; thank you.

Though easiest on the eye + smoothest in hand (maximum speed of movement?),
it is the most challenging to design because its letters go on piling, and consequently — in contrary to a boatload in water—, it is the baseline (surface of water) that has to be moved up and down, necessitating uneven line spacings.
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That's why it is used for boatload headlines but never for text.
http://typophile.com/node/84552

This style is extremally difficult, indeed. I had chosen it as an inspiration, rather than as a guideline, as I wrote... I am not able to manage it.

But I would like to put my concerns aside, and ask another question, which is a part of my problem:

In a simplified typeface, both words - النجوى and الفحوى - have the same form of Jeem and Hah. This situation is self-explanatory. But how would the possible introduction of stylistic alternates change the perception of the typeface? Is it more "human" then or more readable? I can learn some technical rules, but I can't get the insight, to understand them...

Thank you for attaching link to Mr Maqtari's work. Very formative knowledge.