[@Evanist]

You can have your own class, and I don't see any problem withe riding a chocobo into battle. Now the actions that the chocobo takes after you're dismounted will count as a character action, but you could progress to wyvern, pegasi, or more powerful chocobo in time.

As per sharing skill levels, riding a land based creature and air based creature is totally different, so they won't share a level.