An abstracted interface for your datalayer is a must have.

Many startups and projects begin with a single db and grow into new dbs as the business requirements change.

Obviously if you know your data model well enough you can foresee a lot of these requirements and pick the right tool for the job.

However, you will usually need to pick a db to start with and hope that it will accommodate as many of those unknowns as possible.

I believe that Postgres is the best choice in that scenario, as it is extremely mature and has an incredible amount of flexibility.