You might want to consider using Apollo. Especially if you’re going with Drupal (and SQL) for now, and will be integrating Meteor into things later on. Apollo is already quite usable, and will only get more usable in a couple of weeks/months. By leveraging Apollo (and GraphQL) you’ll be able to integrate Meteor with your SQL based backend directly (and won’t need to worry about using Mongo).
Again, I would recommend looking at Apollo before you head down this path, but you can always migrate data between databases by writing/creating a custom data migration process to do so. As long as you can get data in/out of your database, then you can write code to automate the process, and migrate that data into/out of another database. You could also consider keeping your Meteor based app and your Drupal app separate (both using separate databases), and have them communicate with each other through something like the Drupal RESTful Web Services module.
There are also many cool projects out there that make sharing data between Mongo and SQL based databases easier, like Stripe’s MoSQL project for example.