yes, cards like beastking of the swamps can act as one fusion material monster, but the others have to be the exact ones.
Rainbow Neos is a fusion card which you need the Elemental Hero Neos and Rainbow Dragon on the field or hand and use Polymerisation .
Yes, "Future Fusion" can be used for "Rainbow Neos" by sending an "Elemental Hero Neos" and a "Rainbow Dragon" or "Dark Rainbow Dragon" from your deck to the Graveyard.
yes there is a rainbow neos card in japan
Rainbow Neos is a fusion of Elemental Hero Neos, and either Rainbow Dragon, or Rainbow Dark Dragon.
Yes, Rainbow Neos is the fusion
Well, the episode of Rainbow Neos has already been released and there are a lot of pictures of how Rainbow Neos looks like, he has a huge ressemblamce to Rainbow Dragon with the whats on it like the wings, hands, jewels, body, etc. While it's still got the basic shape of neos like all the Elemental Hero Neos Fusions look like.
You use any appropriate Fusion Summon card (ie Polymerization) while you have Elemental HERO Neos and any 'Ultimate Crystal' monster such as Rainbow Dragon, either in hand or on the field.
nope
Yes Rainbow Neos is legal. A player can have 3 copies of this card in the fusion deck in both advanced and traditional formatted tournaments.
Rainbow Neos 'cannot' be special summoned except by Fusion Summon of the listed components. There is no way around this, not even cards like Metamorphosis or Cyber Stein.
Due to a bit of an oversight when Rainbow Neos was translated into English, you cannot use a Fusion Substitute in the place of Rainbow Dragon or Rainbow Dark Dragon, nor can Elemental Hero Prisma take the place of one either. The reason is, the Japanese version says it is made from "Elemental Hero Neos" and an 'Ultimate Gem Lord monster'. This wording is actually non-specific, it is not naming anything specifically, just looks for any monster with Ultimate Gem Lord in the name. You therefore can't use a substitute or Prisma in conjunction with that second component, only Elemental Hero Neos. When they translated the card into English, they just wrote Rainbow Dragon and Rainbow Dark Dragon by name, not realising it changes the way the card is played - but the card has to follow its original Japanese text, and that is how it is ruled at tournaments. So King of the Swamp can be used, but only in place of Elemental Hero Neos.
Elemental Hero Neos, Elemental Hero Neos Alius, Neo Space, Elemental Hero Dark Neos, Elemental Hero Divine Neos, Elemental Hero Flare Neos, Elemental Hero Glow Neos, Elemental Hero Grand Neos, Elemental Hero Storm Neos, Elemental Hero Air Neos, Elemental Hero Aqua Neos, Elemental Hero Chaos Neos, Rainbow Neos, Neo Space Pathfinder, and Instant Neo Space.