Well, if Nomura wanted more indepth like blending the Final Fantasy with KH, he would've not let Aerith show up at all. Why? Some of you are saying that it could be the Final Fantasy prequel before their debut in the Final Fantasy universe. Which would be completely impossible. They would have known each other in the Final Fantasy saga if they had connections to Kingdom Hearts. In other words, Cloud grew up with Aerith and the rest of AVALANCHE, in Kingdom Hearts but supposedly knew nothing about them until he met them again in FF7? Did he lose his memory of the Kingdom Hearts saga that he would go, 'Hey, I'm Cloud. This is the first time I've met you.' to Aerith in FF7, even if they grew up together in KH? It doesn't make any sense. Either way, in the KH universe, they grew up together, so Aerith is still alive or whatever.
Maybe, to stay true to the story, she should have died and the KH theme would have been set to after the Final Fantasy Trilogy where things aren't so complicated anymore. (Plus, they have their Advent Children costumes, so that should show that it's in the future after Aerith died instead of the past.)
In other words, like what Nomura-san said, they are 'guests' which, in fact bear no reality to their home games of Final Fantasy, an AU. An Alternative Universe where nothing makes sense and whatever they do is controlled by Nomura because he rules them all! So, it's best to ignore trying to combine Final Fantasy with KH because it'll just give you a splitting headache and a trip to the asylum.
Happy Trails.