BadBigBen, Yes parallel installs can be done as long as the OS's are all Windows-based or all linux-based (which we didn't know initially) but I still would strongly recommend against it for Windows because when something goes wrong with one OS, it has a way of affecting the others, especially when the systemroots of such a configuration are likely to be named C:\Windows, C:\Windows1, C:\Windows2, etc. Windows applications are better than they used to be, but I would still worry about apps that try to access c:\windows when they should be accessing %SystemRoot%. Multiple partitions is far safer.
mandar1607, why do you think that you can not resize the existing partition? There are many tools available to do just that (as BadBigBen posted). A single physical drive can have many partitions. However, be very careful if your system has a hidden recovery partition - resizing partitions might affect your ability to restore in an emergency.
Will your system boot from an external USB drive? This might be the best and safest solution. You could add multiple partitions on a large external USB drive with the additional operating systems you could ever want. I haven't tried this setup, but I bet it would work fine. The only additional piece that might be needed would be a third-party boot manager.