Is there a way to set interval between "syncs" in Safari Beta

There is no user-interface for this. What kind of controls or options would you expect?

Ove - our model is a bit different than Firefox's, since we don't reside within the browser but are instead a stand-alone background application that is always running. This is because on MacOS, other applications besides Safari can write to the bookmark store; for example, doing a MobileMe sync or connecting your iPhone may update your bookmarks.

You can turn off automatic syncing altogether. By default, we sync every 15 minutes, or within 1 minute of detecting local changes.

If you really want to change the automatic sync interval from every 15 minutes to something else, you can do so manually. This is not official supported and is only for the most advanced users; but hey, here's what you'd do.

Stop Foxmarks for Safari. From your home directory, navigate to "Library > Foxmarks > FoxmarksPrefs.plist". Add an entry: "autoSyncTime" and set its value to the auto-sync time, in seconds. For example, hourly sync would be 60*60 = 3600. Save FoxmarksPrefs.plist. Then start Foxmarks for Safari again.