There is an awesome way to do this in FCP X. First, if you HAVE to record audio to a secondary device (never the best practice), I'd stop audio recording when the camera is not running, but that's a personal preference. But be sure the camera is in fact recording audio, and picking up the interview decently, or you won't have any reference to work with.
Unless you're using very high end hardware, you probably can't sync timecode between devices. The only way to do this is using a Timecode Generator, and running cables from it to all of your devices, who then read that timecode and write it to their files. But again, only high end hardware has this ability. It is a dedicated port/cable.
All due respect but with your specific camera, since you have XLR inputs and nice mic preamps, you'd be insane to not record audio to the camera. Even a Canon 5D MkII records GREAT audio with a JuicedBox. A dedicated audio device won't improve your audio, only improving the mic will do that. Only if you can not physically have the mics connected to the camera should you use a dedicated audio device. There are affordable wireless systems to help with this. Recording audio to your camera, especially one that nice, saves you lots of time, drives space, and headaches.
So if you must use a separate audio device, in FCP X, you'd select all relate clips in the Browser (Command-click). Oh, and be sure you Optimize your video on Import for faster more accurate workflows. Once they are all selected, right-click on any of the selected clips and chose "Synchronize Clips" (Option-Command-G).
This will create a specialized Compound Clip called a Sync Clip.
Double-click it, or right-click and chose "Open In Timeline" and tweak anything that isn't 100% synchronized.
In the Browser, with the sync clip selected, go to the Inspector > Audio > Channel Configuration and configure all of the audio channels as you need.
The FCP X course on media ingesting (FCP X 102) covers syncing in more detail.