I have an existing CentOS 6 installation working wonderfully and its owner now wants to add a separate Debian installation onto the same machine. He only accesses this host via the command line and via a NoMachine VNC-type GUI connection, since it's located in a data center and thus out of reach.

My questions:

1. Can I create a "hot partition" on the existing CentOS 6 installation without destroying what we have now?
2. If I can safely add a new partition, is there any tool or method the user could use to choose which OS it will boot into? Something like a boot picker. Remember he does not have physical access to the machine.

I'm coming from a Mac/Windows background and thus don't know the answers.
Suggestions or ideas are welcome.