Be very careful here. The operating system and applications use a certain amount of space for housekeeping (so to speak). You are over-simplifying how your storage space gets used.
You are the victim of some pretty bad information, so before doing anything rash, take a moment....
If you have a Retina MacBook Pro 13", your computer is 100% compatible with OSX Mountain Lion, not sure where you were told otherwise.
Also, a clean install of the operating system likely won't change anything. At best, it might recover a small amount of storage, but it will most likely provide zero benefit after you finish restoring applications, etc...
So... take a breath, and get some better information. I would start with installing a utility to help you correctly understand how your drive space is being used. I use Spacie (http://spacieapp.com/), which costs a few dollars from the App Store, but does a great job of showing what precisely is using up space on your SSD.
finally - as long as you don't plan on storing video or music files (or lots of virtual machines, or other huge files), the space you have should last quite a while. If you *do* plan on storing these file types, then you need to arrive at better strategy, because that internal SSD will never be enough, no matter how carefully you manage the space.
Thanks to the info from Kappy, we ordered the MacBook Air with the 128GB SSD for my daughter to use in college. So far, she still has tons of free space, and at her current "run rate" it will take years before she starts to run out of room.
Message was edited by: griz_fan
Oh - one more thing. Get Time Machine setup. I can speak from personal experience that this is VERY valuable. A 500GB external 2.5" hard drive would be ideal, and can save you a LOT of grief down the road (I'm speaking from personal experience here)