So let's say that not all 10,000 Pokemon are in X/Y - then how do you get the old ones into the game? If I remember correctly in the DS games you would insert a GBA cartridge to import older Pokemon into Pal Park. As far as I know, there isn't a way to have 2 games inserted into the 3DS at once, so how would you get access to the Pokemon not present in X/Y?
Most likely you would miss out on the Sapphire/Ruby/Emerald Pokemon since those were the only GBA games. Red/Blue and Gold/Silver pokemon are in SoulSilver/HeartGold. Then you have the other DS games, Pearl/Diamond/Platinum, Black/White, and Black 2/White 2.