Actually, I don't think Starfleet would lower their rank requirements to the point of putting a lieutenant in charge of a Saber or Defiant.
At the very minimum, they'd probably take a lieutenant commander and brevet him to the rank of commander until he achieves the permanent promotion.
For a Steamrunner or Norway, a commander can fill the role. Rather than brevet a lieutenant commander to a ship of this size, Starfleet would probably just promote the officer early.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz