I had already installed linuxmint choosing option use entire harddisk
in partition manager and i want to have fedora along with linuxmint the problem is i don't know how i can use the free hdd space for fedora and also if i install fedora then can i see the option to choose any one of the OS between system starts help me

You can install multiple operating systems within the same partitions if the operating systems are the same type as in all windows......or....all mac.......i am not sure about linux products though. The problem with this type is that they auto share the same system folders and files which in turn becomes a mess of confusing issues.

It is best to use separate partitions. It is better to use different drives all together.