It's better to install it on a separate partition so if anything goes wrong you can always startup from the os9 partition.
You can change your startup disk in the OS 9 control panel or in system prefs in OS X.
You can also choose from wich disk you will boot when you press option during boot.
When you want to use Classic it recommended by apple to first install OS9 on that partition.
After choosing the disk on wich you want to install, you can choose wich parts of 10.2 you want to install, use the "special" button. Uncheck everything (languages and printer drivers) you won't need.