Mac OS X can be compatible with Classic OS files (such as type/creater, icon, resource fork, etc) to a certain degree.
But a system set aside to boot and serve OS 9 would be a better idea. It just ... works better.
Given that you wish to use Leopard, you can. 10.5 will not network to anything under 9.2. There is no need to reformat anything. The operating system itself manages Classic OS file data: journaled HFS+ is perfectly fine and has no bearing on the management of the file data.
But a system set aside to boot and serve OS 9 would be a better idea. It just ... works better.
Given that you wish to use Leopard, you can. 10.5 will not network to anything under 9.2. There is no need to reformat anything. The operating system itself manages Classic OS file data: journaled HFS+ is perfectly fine and has no bearing on the management of the file data.