If I brought Groudon from my SoulSilver, can it evolve or does it have to be the one from Omega Ruby?
In order for Groudon/Kyogre to revert to their Primal Formes they need to hold the Red or Blue Orb, respectively. The Orbs are basically Mega Stones, in that a Pokémon merely needs to hold it in order to transform. In order to Mega Evolve Tyranitar or Altaria, they do not need to be from Gen VI. Likewise, Primal Groudon/Kyogre can also be from any Gen. (The same goes for Mega Rayquaza. It only needs to know Dragon Ascent.)