The only concern I have with making the Wyvern a companion, is that it would link the Wyvern to the party list. Meaning that if you had 8 people in your party, you could not pull out your Wyvern. Additionally it means that you can't use your Wyvern in Dungeons. Part of the whole point of having a Wyvern on a Dragoon is that the people asking for it want to be able to use them in their dungeons. Or at least I would assume so anyways.