I'm pretty sure that the installer does not touch the partition layout.
Remember
Android is weird. I wouldn't be surprised if some devices/bootloaders have specific expectations about the partition layout
these are embedded devices. If the bootprocess is severly messed up, you can't exactly put the hard disk into that other device and fix the partitions
So, while I guess it is possible to make cache smaller, I am not gonna try on my device and I'd caution you to do so!
Now to the aspect of "I'm running out of space" due to tinkering with libertine, ... well you still have some 6GB to through around, not huge, but not nothing either. If you run out of space, rather than doing acrobatics with partition sizes you want to look into acrobatics with bind mounting