Short answer: leave it up to the system.
Programs installed via Software Center are installed in system folders under the / folder. These system folders are defined in the software packages. In order to change these folder paths, you would need to repackage a program.
When working with different partitions for / and /home, you must make sure to provision sufficient on / for the software you want to install. In principle, the 30 GB you have foreseen typically is well enough to accomodate the system and plenty of software.
To control and accommodate allocation of where software is installed yourself requires sufficient technical knowledge of the system
- As already indicated, to change the actual installation paths requires repackaging the software.
- Alternatively, you can always connect different partitions to different parts of the root file system through symlinking. In the old days of unix, where drives were small,
/usr typically was on a separate partition. That is why we still see both /bin and /usr/bin folders, which only in very recent times were actually "unified" in Ubuntu by linking /bin to /usr/bin.
Programs that you install manually outside of the software sources, should be installed under /usr/local or under /opt. Programs that come with a setup script quite commonly install under /opt. For example, Google Chrome is installed under /opt/google. To place an executable in the search PATH, a symlink is placed under usr/bin that link back to the actual executable /opt/google/chrome/google-chrome.
