Fluros of Myra's Marvels

Fluros’s middle ability triggers only once if you cast a creature or Vehicle spell with power and toughness both equal to the chosen number.

Fluros’s middle ability will overwrite any previous effects that set the target’s power and toughness (if any) to specific numbers. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.

If Fluros’s middle ability causes a Vehicle to become an artifact creature, it doesn’t count as “crewing” that Vehicle for any ability that would trigger off of a Vehicle becoming crewed. That Vehicle will become an X/X artifact creature, not whatever its printed power/toughness is.