You can substitute this card for any 1 Fusion Material Monster, but the other Fusion Material Monster(s) must be correct. You can Tribute Fusion Material Monsters on the field, including this face-up card; Special Summon 1 corresponding DARK Fusion Monster from your Extra Deck.
Ah, so Hex's second effect only counts as a Special Summon and hence it doesn't work with cards that specifically require fusion summoning...
... The person who spent the time to write the literal wording traps into these cards needs to get laid.
So, I get that when it's on the field with another 'correct' card you can fusion without poly. Can you however use it WITH poly from your hand as a sub for fusions vs the "on the field" requirement?
The Dark - Hex-Sealed Fusion / The Light - Hex-Sealed Fusion / The Earth - Hex-Sealed Fusion: This monster's second effect cannot be used to Special Summon Fusion Monsters that can only be Summoned by Fusion Summon, such as "Dark Paladin".
Doesn't work for Dark Paladin. Maybe B. Skull Dragon, but that still requires you to have either REBD or Summoned Skull on the field. Otherwise, you might as well be using any other fusion material substitute.