As of right now, none. Since Isshu is nowhere near any other regions, there will only be new Pokemon (at least until you defeat the Elite 4).
Yes, just not with any versions older than that
You have to trade across from older versions or from someone willing to trade
You have to trade them from older versions of Pokemon.
A trade is required from older versions
No. Pokemon can't be traded from Black/White to older games at all, even if those pokemon existed in those older games.
Questions like this are always asked. You can't catch this Pokemon in diamond or pearl versions of the game, only in the older fire red and leaf green versions. They can however be transferred to Diamond or Pearl once caught by going to Pal Parkafter you beat the Pokemon league.
sprout tower or trade from older versions
yes
No, you cannot trade Pokemon from White version to SoulSilver. You can never trade Pokemon from newer versions to older versions as they are not capable of comprehending what those new Pokemon are. The reverse trade will probably be possible, however.
No, you can only have one save file, just like in the older versions.
you cant get the new Pokemon in any of the older games
You'll have to trade for it or transfer it from the older Pokemon DS games.