OS 9 and Tiger will live on the same drive just fine.
If you have an OS 9 CD put it in the drive and reboot. When you hear the chime press the C key. The Mac will boot to CD and allow you to run the OS 9 installer.
* DO NOT INITIALIZE THE DISK
* BE SURE TO DO A CLEAN INSTALL
Once installed you can use the Startup Disk preference in OS X or the Startup Disk control panel in OS 9 to select which OS to boot to. Additionally you can press the Option key after the boot chime to select on OS as well.
* note from Apple
Classic Compatibility
Tiger's Classic environment requires:
Mac OS 9.1
or
The minimum Mac OS 9 version natively supported by your computer, if any
...whichever is later.
Tiger does not include a version of Mac OS 9. Mac OS 9.2.2 is recommended. Generally speaking, you can use the version of Mac OS 9 that came with your computer. If your computer shipped with Mac OS 8.6 or earlier, you can purchase a copy of Mac OS 9 by calling AppleCare or Apple's Telesales group. (Mac OS 9 is not available to the general public on the Apple Store online.)
The version of Mac OS 9 available from Apple's Telesales group will only install on your computer if your computer can start up from Mac OS 9. Computers that can't start up from Mac OS 9 all come with Mac OS 9 as part of the restore bundle.
Classic requires QuickTime 6.0.3 (or later) in the Mac OS 9 System Folder (for use by Classic). If you need to upgrade QuickTime for Mac OS 9, you can get QuickTime at
www.apple.com/quicktime/download/.
Tiger itself comes with QuickTime 7 for use by Mac OS X-native applications.
Most Mac OS 9-compatible applications are compatible with Mac OS X Classic. Check with the developer of your application for information on Classic compatibility.