I think Izunabi meant that all non specialists have 0% affinity with specialisation, not just Kurapika's case, so unless one is under the specialisation category, one cannot develop specialisation Hatsu. If Palm had her nen category changed to specialisation, then at the time when she used Hatsu on Coffee it wouldn't have overflown, but something else would've happened.
Yes, all people not only kurapika have before they actually develope such an ability.
I do think there is a misunderstanding about kurapika and izunavi: Most ppl think kurapika changes his nenclass when his eyes turn scarlet, but i think kurapika keeps being a plain old conjurer and the scarlet eyes are just a requirement for him to use that hatsu.
I know he did the water test and it said that he was a specialization user at that point, but i would argue that the water test is only accurate for beginners of nen.
It seems out of question for me that people like biscuit can do with that water whatever they want to.
It is only accurate for beginners because they usually can only do things in their dominant nenclass, if anything, thus you only see that effect.
I think anyone who is somewhat proficient in nen and is only one class away from manipulation could move that leaf a bit.
Kurapika, in the state he was in, simply used specialization nen without knowing it (once his eyes turn scarlet) and thus something else happened with the water when he applied his nen to it.
Thus, my guess is it goes like that: Every person has a most dominant nenclass.
Thus class dictates two things definitely: Roughly how good you are at other classes (minus maybe specialization) and how likely you are to learn specialization skills later (if you are not a specialist initially, which seems rare regarding izunavi's explanation).
However, once you develope specialization nen (which every nenclass can do, at least izunavi only spoke of likelyhood, not that it is impossible for enhancers), all bets are off.
I assume it corresponds with affinities, but that is not stated in the manga.