Expansion jobs can be unlocked prior to beating the expansion story. Viper can be unlocked at level 80 by players who haven't even finished the ShB base story yet. Blue Mage was introduced in Stormblood but could be unlocked by players as soon as they had beaten the Ultima Weapon. Beastmaster will almost certainly be able to be started by players before they reach the Regulator reveal, so it likely will not feature that lore.