No. You can have a merlin router and stock or Merlin and older version of Merlin or older version of stock. But know with any newer version comes bug fixes and security updates so regardless of stock or Merlin I’d always suggest keeping your router up to date to prevents issues with changes between versions.
Models don’t matter as long as they support AIMesh. That said you don’t have to use AIMesh as it has more generic modes for non asus routers like AP mode.