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 Rainbow Neos is legal. A player can have 3 copies of this card in the fusion deck in both advanced and traditional formatted tournaments.
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.
Rainbow Neos is a fusion of Elemental Hero Neos, and either Rainbow Dragon, or Rainbow Dark Dragon.
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.
Technically,in the traditional sense, it is classified as an Elemental Hero as it is an Elemental Hero Neos fusion, but as the card does not include the term 'Elemental Hero' it may not be classified as one in games, both virtual and real. Hence effects that affect E-Heros do not apply to Rainbow Neos.
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.
nope
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.
No, a "Cyber End Neos" card does not exist.
The only way to summon it is by the listed text on the card. However you could use it in a deck without the Crystal Beasts in a Rainbow Neos deck.