Support multiple order types in core

The code you've provided isn't sufficient, because there's much more that would need to be done to support multiple order types in core. Not only would we need to take care of the field interfaces, but we'd also need to take care of it in the Cart module specifically and potentially in other modules like Checkout. I'm making this a feature request but postponing it, as this type of change won't get in before 2.x. I've added it to #1467180: Open 2.x Drupal Commerce branch as a reminder that this is a topic needing consideration.

Initial implementation of it (let's say, a fundament for it) is done in #1509994: Missing entity property info results in notices / WSOD / fatal errors during installation.
There you could find hook descriptions and implementations for hook_commerce_order_type_info and hook_commerce_order_type_info_alter that are (obviously) necessary to support multiple order types.

Is there any way to implement this in core?
Ther´s already Commerce a contrib Order types module, maybe it can be merged?

Not in the 1.x branch; we intentionally left it out, but see my comment above about 2.x.

Thanks!!

I think this may already be taken care of?

Correct. There's a full UI, now we just need to add checkout into the mix.