Special Summoning the Fusion Material Monsters is an option. So you may use "De-Fusion" to return a Fusion Monster to its owner’s Fusion Deck even if you have only 1 or none of the appropriate Fusion Material Monsters in your Graveyard.
If you have only 1 of the Fusion Material Monsters in your Graveyard you cannot Special Summon it.
You may use "De-Fusion" to Special Summon "Goddess with the Third Eye" or "King of the Swamps" if it was used as a Fusion Material Monster for the Fusion Monster selected with "De-Fusion".
After you attack with a Fusion Monster, you may activate "De-Fusion" in the End Step of the Battle Phase to Special Summon the appropriate Fusion Material Monsters and attack with them in the same Battle Phase.
If you activate "De-Fusion" and select a Fusion Monster that was Special Summoned by "Magical Scientist" or "Metamorphosis" you can return the Fusion Monster to the owner’s Fusion Deck, but cannot Special Summon any Fusion Material Monsters since none were used.