If you didn't do the Ruddle and Rumina sidequest earlier in the game, then there is no way to recruit Izak (you get the Blueprints from that sidequest, like Valkyrie said). The AI Disc is obtained (in a set of items) by one of the Eternal Sphere Programmers in the Sphere Company Building before you fight Belial and Beelzebub, so I'm not completely sure if it's missable or not.

However, contrary to what you may have heard (read in the Brady guide or from elsewhere), you do not need to to the sidequest to unlock R and R's shop in Aquios later in the game. You just need to go to the upperleft-most house in Aquios, and the best purchasable main-game weapons and armor are there. I find those adequate, as the equipment in the post-game dungeons are much tougher than the ultimate IC equipment anyway, but if you must stay on top of the game, then that's fine as well.