Thank you all for your kind and prompt responses.
Judging by what Randal mentioned, a true flex nib is out of the question. It thus seems that I will need to visit a shop somewhere to test-drive a few of the nibs with a limited amount of flex in order to determine whether any such one would fit my needs. Another question then lends itself to this discussion: how much pressure does one need to (reasonably) flex a "semi-flex" nib compared to a "true flex" nib? That is, how much "stiffer" is the semi-flex? I am sure that there is a variation depending on the brand etc., but if a general conclusion can be made, please, enlighten me...
As for italic nibs, this is an interesting idea. I will need to research more on this, especially on the differences between -- say -- Lamy F or EF nib and their 1.1, 1.5, and 1.9. Some information can be found e.g. on PenInkCillin (http://peninkcillin....-11mm-15mm.html), but from there it seems that the difference between 1.1 and 1.5 (or 1.5 and 1.9) is quite significant with no "middle ground" being available. In other words, either I would get something almost identical to an F nib (1.1), or a nib that produces very broad lines (1.5 and especially 1.9).
P.S.: The target end result is something like the Waterman sample in post #92 of the thread http://www.fountainp...ue-black/page-4 (I would not mind a slightly greater width variation if inevitable, but not too much of it).
Edited by nvx, 21 October 2013 - 08:41.