How can I control the frequency of synchronization with Google?
I don't know of any way to set the sync frequency or to make syncing low-priority, unfortunately.  You could use Locale to turn off/on auto-sync based on profiles you set up, however.  There's a Locale plugin that might do most of the work for you: https://market.android.com/details?id=net.andvari.android.syncsetting