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.
No, they (at least the more recent ones like Nerokius) pretty clearly say they first have to be Summoned with the listed method. (And since it's not a Fusion Summon, you can't use them as regular Fusion Subs for these Summons, either.)
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?