There is no "red obelisk the tormentor". There is a TCG-legal version of Obelisk the Tormentor, released in Shonen Jump (see http://yugioh.wikia.com/wiki/Obelisk_the_Tormentor_%28Effect_Monster%29).
Yes, unless specifically mentioned on the banned list (Exchange of the Spirit, Victory Dragon, etc) or is the JUMP-EN004 Obelisk the Tormentor, you can use the other Shonen Jump cards in a legal deck.
You can use Obelisk The Tormentor in a duel ONLY if it is the one released in an issue of Shonen Jump (Card ID will be JUMP-EN037). This card has an effect listed, and the back of the card is normal so it cannot be considered marked.
legal 'Obelisk The Tormentor' came out in the Jan 2010 issue of Shonen Jump, and legal 'The Winged Dragon Of Ra' came out in the Jan 2011 issue of Shonen Jump so im only assuming that the legal 'Slifer The Sky Dragon' will come out in the Jan 2012 issue of the same magazine.
JUMP-EN037 from Shonen Jump Magazine is a totally game legal version of Obelisk the Tormentor and can be used in tournaments. Any other version cannot be used - this is actually stated on the card, in English on JMP-004, and Japanese on GB1-002.
No, he has only ever been a video game promo, or available as part of a Shonen Jump Magazine promotional offer.
Yes - Shonen Jump last year released a tournament legal, Effect monster version of Obelisk the Tormentor. Winged Dragon of Ra has likewise been released recently. Slifer however has not been announced yet.
Obelisk the Tormentor has been distributed in the following card packs - Gold Series 4, Battle Pack 1 & 2, and Premium Gold. At other times he has been limited edition inside games, special sets and once he was a reward for Shonen Jump yearly subscribers.
Both the original print and the legal Effect Monster version of Obelisk the Tormentor was released as a promotional card in a Shonen Jump Magazine.
Obelisk the Tormentor is not banned, you may use three in your deck. That's the legal 'Effect Monster' print from Shonen Jump. The original print was never actually legal in the first place, it was strictly promotional, had an illegal back, and even said 'This card cannot be used in a duel' on both the Japanese (GBI) and English (JMP-004) prints.
I have the GB1 Ultra rare set, two of the GB1 secret rare set, the JMP/DOD/YMA set, and the most recent Obelisk the Tormentor from Shonen Jump.
Since they released Obelisk and Ra, it stands to reason that Slifer will follow. It'd be very odd if he didn't.