You may compare them in all their stages.
Marluxia has 4 stages (CoM Illusory Marluxia, Marluxia on his robot, Marluxia with Grim Reaper, and Marluxia replica from KH2FM)
Seymour has 4 stages (Temple Macalania, Bevelle Exit, Mount Gagazet, and inside Sin)
No, you cannot use Marluxia + KH moon...