I was wondering if you could make 6-limbed creatures; Like a dragon, a gryphon or a 6-footed lizard. Would it affect the gameplay?
Well, of course you could make 6-limbed creatures. See the class Insecta.
How would it affect gameplay? You have six limbs. You walk a bit more. You use more energy when moving, and require more energy as upkeep for your body.