You can never tribute an opponent's monster unless the card specifically says you can (Lava Golem, Soul Exchange).
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When using metamorphosis the LV of the Fusion monster should be the same as the LV of the monster you are going to tribute.
It requires 1 tribute
A tribute summon is a form of Normal Summon. So that means you can only tribute summon OR normal summon ONCE per turn.
Assuming the defending monster is also in attack position, both monsters will be destroyed.
Five-Headed Dragon.