I strongly recomend placing os x and os 9 on different partitions, simply because if either needs to be re-installed, you can just format that partition.
If they are on the same partition, you can't uninstall os x.
Its also worth noting that most programs with unix or linux heratige will insist on being installed where the rest of the system is - that is,
/applications
or /usr
But like someone has allready said, you can allways make symbolic links and therefore place your apps on a different partition.
Also, don't try and place os x on anything less than 2.1 gb. Thats what you will need if you install the developer tools (strongly recomended).
I ended up with: 5gb os x, 1gb os 9, 34 gb extra (for my applications and mp3/mpegs). Yeah, thats 40gb.
Hope that helps.