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.
This card can be used as a substitute for red eyes b. dragon, but not for a generic warrior monster. The thing you're doing wrong is trying to fuse 2 re-eyes b. dragon together. You need a warrior on the field to make it work.
The Hex-Sealeds have two effects. You can use them as a regular fusion substitute with Polymerization (like you would Versago). Or you can Tribute them and the other materials for a Fusion from your field to Special Summon the Fusion.
Keep in mind that second effect is NOT a Fusion Summon, though. So you can't Summon something like Shining Flare Wingman that has to be Fusion Summoned.
I couldn't FUSION-SUMMON (w/ polymerization) Buster Blader the destruction swordmaster usind this card and an Explosive Dragon in my hand as Fusion Material... anyone knows why? :'(
Are you trying to summon the BBdragon swordmaster? Of course you cannot, because the BBswordmaster is NOT treated as Buster Blader in hand, BUT you can use that dark he + BBswordmaster to Red-Eyes Slash Dragon
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?