I was able to overcome my Google Music space issues using the instructions in
this thread. The first time I tried it, Google Music was creating its file directories under /sdcard/Android/data/com.google.android.music, and a symlink couldn't be created there. The accidental solution was to use leapinlar's SD Swap file, which doesn't work very well for its intended purpose, but I think it made the internal SD card inaccessible to the Google Music app. So, when I cleared the app data and restarted the app, it created its file directories at /data/data/com.google.android.music, where a symlink works just fine. I flashed the undo file for the SD Swap and placed the symlink, and now all my downloaded music is routed to my ample SD card
Now I'm just trying to replicate those results on my phone, but no luck so far :/