How to install Mac OSX Mountain Lion as virtual OS in Mac OSX Yosemite? I have Macbook Pro ( late 2011) which I recently upgraded to Yosemite and works fine. It has only one hard drive which is 256GB SSD (Samsung). After purchasing I received Mountain Lion as an app in Apple Store and it is downloaded and Apple is not going to ship physical CD.

I want to install Mountain Lion as virtual OS within Yosemite using VirtualBox because VirtualBox is free. Is it possible to do that without making my system unstable (a risk I cannot take) ? Does any one had issues with this type of setup or anything I should be aware of? Is there another more stable but free alternative to VirtualBox (other than Apple Bootcamp but I do not like having to shut down my primary OS and rebooting to Mountain Lion) ?
It shouldn't make your system unstable since it runs in its own "box" per se.. I have done it for other operating systems such as Ubuntu, Windows, and older versions of OS X.