Alright, guys. The major symbols we see in KH:
Heartless: A heart with a shape similar to a fleur-de-lis on the bottom. Two cracks form an X over it.
Nobody: Basically an upside down version of the Heartless symbol. The 'cross' is enlarged and the heart shrunk.
Spiked Wheel: An X with a spiked wheel circling it. Used (as others have said) to denote enemies that aren't Heartless or Nobody (and not used in-game, I believe; rather, in the strategy guide, etc).
Cross: Seems to be inspired from the Nobody symbol. Most prominently, it is seen on Roxas' zipper (not necklace, by the way).
Crown: A crown. Seen most prominently on Sora's necklace, and the sequence for opening gates to other worlds. Mostly just a nifty design.
Heart: A heart. Seen most prominently in the game's logo. Seems to just be a design with little actual symbolism.
Conclusion:
Only one of those in the picture in question is actually an enemy symbol. My guess is that this design was put together this way just for looks. Sorry, but there just doesn't seem to be any game-relevant significance to it. ^_^'