There are two formats, Standard and Expanded, that now exist in the Play! Pokemon format. You can see which sets are legal here:
http://www.pokemon.com/us/pokemon-news/play-pokemon-2015-season-format-rotation/
As it says, additional expansions, such as the recently released Phantom Forces, become legal 21 days after release.
Older cards reprinted in later sets can be used, although in some cases there is errata and a reference card is need. The folks over at PokeGym maintain a list of these that is quite helpful. http://pokegym.net/forums/showthread.php?41415-Standard-and-Expanded-Format-Legal-Card-List-Discussion
Darkrai EX is one of those cards that is still in standard because even though it was introduced in Dark Explorers, it was reprinted in Legendary Treasures. So, you can definitely base a deck on those two! Have fun!