I was just using Kingdom Hearts to make a point about what kind of cross-overs work. Kingdom Hearts took the characters and kept what they looked like (to some extent), kept their personality and set them in with a whole bunch of different situations.

In Dirge of Cerberus it's the same world of FFVII. The same people exist and the same things happened. The only way they could bring in other characters and have it fit well would be to make a character who looks and acts like and is named Zidane (or anyone else) but have no background as the character Zidane was in FFIX.

That'd be lame.