We know for a fact that a lot of the DY pilots are SK and BU rejects and therefore not well suited for this "aptitude-test-thingie". So would it be fair of DY to ask skills from the newbie's that their old-timers doesn´t possess
You're assuming that these "rejects" were fired because of their lack of skill, which i think it's safe to say had nothing to do with it. All pilots accepted into SAS / BU met the required standard, i would assume that seniority etc would play a bigger role in who got fired.
There's really no way of telling how a person will do in these tests (at least with "proper" tests like they use in the air force etc), so there's really no way of knowing if the "old folks" would do better or worse than the "new guys". And why would they have to test every pilot currently employed by Norwegian? We're talking about testing applicants, not current staff. Perhaps it would be more "fair" if everyone, including currently employed pilots, had to undergo these tests, but like several previous posters have already said; "fair" doesn't enter into the equation.