Normally you’ll tune the skew after you tune the length of your differential pair, as the skew comes normally from the fact that you mostly won’t have the same amount of left/right bends, which would make the length of the signals unequal.
tuning the skew shouldn’t make the pair longer, as you (in theory) just match the shorter wire in the pair with the longer (and length matched) one. obviously you should check at some point that the longer wire is actually length matched with the other pairs.