The cost reduction applies only to generic mana in the total cost of legendary creature spells you cast.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of Serah Farron or Crystallized Serah's first abilities). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

Serah Farron's last ability checks at the moment it would trigger to see if you control two or more other legendary creatures. If you don't, the ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you don't control two or more other legendary creatures at that time, the ability won't resolve and none of its effects will happen.