PS: in alpha sapphire you can catch palkia you know dude!? all pokemon in official battles must be 6th gen now so you can catch everything in ORAS with the exception of 1 legend and I can't remember what it is, it's manaphy maybe. (this is when the have the hexagon, all pokemon from XY and ORAS (and giveaways during this time) have the hexagon mark. thats why I asked if you need a 6th gen or not. it only truly matters if you were gonna use the pokemon to enter a competition, you can still use previous gen stuff in game and battle online and stuff like with them.
You can't use the legends in the official battles or competitions, it's not the reason why every legend is in the game. It's clear that RBY, GSC, DPP or BW will not recieve remakes this gen, that's why they put all the legends and starters in XY and ORAS. You can use 3rd, 4th and 5th gen Legends in free battles even on ORAS.
The last competition in ORAS was
Generation Showdown. That competition actually allowed you to use Mewtwo, Lugia, Ho-Oh, Kyogre, Groudon, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem, Xerneas, Yveltal or Zygarde. I'm not sure if they had to be from 6th gen though.
If you want to use the Legendary Birds or Beasts, the Golems, the Eon Duo, the Lake Guardians or the Forces of Nature on the Battlespot official battles or in Competitions they will need the Pentagon ofcourse.