Is it possible to go from pre-installed Mavericks to Mountain Lion on my new Macbook Pro?
According to Apple, you should not install previous versions of OS X than the version that came with your Mac. Since your Mac came with Mavericks, you should not install Mountain Lion. But if you want ML, then it may be possible with the following caveats.


  Both PowerPC- and Intel-based Macs ship with a certain version of Mac OS X (or Mac OS X Server) on their installation disc(s). You should not install a version of Mac OS X earlier than that which came with your Mac.
  
  If you install an earlier (previous) version of the Mac OS X than what was included, your computer may exhibit unexpected behavior such as:
  
  
  The trackpad or mouse may not respond properly
  The computer may stop responding
  Sleep/wake issues may occur
  The display image may appear to "shrink" with black bars around it, may appear tinted, or have other issues
  Loss of built-in audio
  Loss of Bluetooth or AirPort functionality
  May not start up past the Apple logo
  


Source: Apple KB — Don't install a version of Mac OS X earlier than what came with your Mac