if you are a startup you are not even considering db2 or oracle. you consider exactly what the op suggested. further, as your business grows so to will your needs. you may start with one backend in a particular configuration and evolve and/or migrate to a different configuration/platform.
Agreed. I love developing locally with a version controlled SQLite database and then deploying to *sql and having everything just work. Sometimes there are differences that need to be worked around (same as cross platform development), but starting with the assumption that your db might change is a good thing, in my opinion.
> if you are a startup you are not even considering db2 or oracle.
Why not? The free versions exceed postgresql. If you succeed and face heavy load, then you just throw some money at the problem rather than blow hundreds of man-hours fidling with the hacks people use to scale the free DBMSes.
You can just throw money at Postgres problems and make them go away. There are a number of very capable firms that do nothing but Postgresql support. And, they don't hold you captive w/ licensing. And if your needs are really that unique they can help you with building extensions to postgresql that support the capabilities you need.
abstraction is "the right thing to do" (tm).