This won't be a supported action. While a minimal installation disc would typically be used to build up something anew while working with that memory constraint, the last one released that supports 32-bit PowerPC architecture was for 12.04. While a mini.iso image for 14.04 does exist it isn't even referenced in the documentation.
As noted on Xubuntu's system requirements page:
To install or try Xubuntu within the Desktop/Live DVD, you need 256 MB of memory, if you are using the Minimal CD, which uses the non-graphical Debian Installer and downloads packages as you install, you need 128 MB of memory.
Once installed, you should have at least 512 MB of memory.
You can attempted the installation but your results may not necessarily be satisfactory in terms of performance. Using the minimal installation disc you can choose what mix of packages you wish to install in case you do not wish to fully install all that Xubuntu ships. There is currently no testing done at all on PowerPC architecture for Xubuntu-related packages.