How many partitions u have now available?
If u free space partition u allocate for the RHE Linux is available, i would advise u to check if it's and extended partition. If it's and extended partition, u will not be possible to install an os on it. It cant boot.
U can check using diskdruid, or if u have "SystemRescueCD" then u can run "qt_runpart" a tool similar to partition magic.