Will iCloud sync interfere with Gmail Contacts/Calendars sync?
With iCloud, you can choose which areas to sync and which not to.  However, if you do choose to sync contacts and/or calendar, I believe you will simply end up with some double events on your phone, which can easily be cleared up by disabling view of the extra calendars.

In short, syncing contacts, calendar, music, and photos are all separate and can be enabled/disabled as such.

And yes, it does, in some way offer you additional redundancy as the Apple cloud and Google cloud are likely housed in different data centers...but this would require you to keep both calendars in sync, which I don't think will be easy to do without being annoyed absurdly.