fiora legend ability triggers if i play a unit that is already mighty

No — Fiora Legend only triggers when a unit that's already in play changes from below 5 Might to 5+, and a unit that enters play already Mighty never makes that transition on the battlefield (Rule 709; FAQ #5835).

"Becomes Mighty" requires an actual Might change while the unit is a game object in play. When you play a unit that's already Mighty, there is no moment where its Might goes from less than 5 to 5 or greater — so Fiora sees nothing to trigger on. This is the same reason playing a 4 Might unit onto Trifarian War Camp doesn't trigger her, whereas moving an already-in-play 4 Might unit there does.

A separate trigger (like Sett's When You Play Me buff resolving after he enters as 4 Might) would trigger her — but simply playing a unit that enters already at 5+ Might will not.