Either a 3DS or 2DS will be fine, as long as it's able to play 3DS games and access the 3DS eShop. However, from my understanding, both games will have to be on separate consoles. So say Blue is on your 3DS and Silver is on another 3/2DS. You can't trade with them if they reside on the same console.
And yes, the original 151 can go on RBY, but the first two generations (251) can go in GS. You cannot trade gen II Pokemon (like Umbreon or Slowking, even though they evolve from gen I Pokemon) to RBY, nor any Pokemon with moves that were introduced in gen II (Faint Attack, Sandstorm, etc). You can trade back and forth within these limits. Also, Pokemon holding items cannot be traded to RBY because Pokemon could not yet hold items then. However, when trading Pokemon from RBY to GS, they may have an item when they make it into GS.
