You an approach a flight manager or fleet support manager and ask and they may protect you with no credit. However it is at their discretion so I wouldn't hold your breath. You may get lucky but bear in mind once you've asked and its refused you've had it.
It is not seniority related and decided upon on each case.
Leave isn't on seniority either but leave points accumulated. School holidays get low points non popular time high. The higher your leave points on a status list the higher your leave choice. Not sure if you join with 0 or average (a la reserve).
NN