You can Set (in face-down Defense Position) a monster that requires a Tribute by meeting the requirements to Tribute Summon the monster (i.e. tributing monster cards).
If the Level 5 or higher monster would be Special Summoned by a card effect, it cannot be placed face-down unless face-down Defense Position is explicitly given as an option (e.g. Spear Cretin), and therefore cannot be Set.
If the Level 5 or higher monster would be Special Summoned by a card effect, it cannot be placed face-down unless face-down Defense Position is explicitly given as an option (e.g. Spear Cretin), and therefore cannot be Set.