You can still get symmetry even with 13 characters as Mickey needs to be just moved down to the bottom group while either the Data-guys flank Aqua on both sides OR you move Terra and Ventus down to be together with Aqua and the Data-guys take the spots currently held by them.
This would also result in a "pyramid" of the games with KH 2 at the bottom, BBS in the middle and Coded on top.
This would also result in a "pyramid" of the games with KH 2 at the bottom, BBS in the middle and Coded on top.