I was thinking about picking up a Colt AR from CDNN.  But, it has a fixed front sight.  Will standard optics mounts put the sight out of view, or would I have to pick a 1x optic to match the set up?

As said, with any sort of magnification your eyes ignore the front sight the way your eyes ignore your nose (which you can always see).  If you're worried about the FSB with a non-magnified optic then go for 'absolute co-witness' and put the dot on the blade as though you were aiming normally.

You really need an adjustable stock with a magnified optics, otherwise you can't get the proper eye relief when shooting from different positions.

Red dot sights work great with fixed or adjustable stocks. I started out using co-witnessed mounts and recently switched to lower 1/3 and feel it's best for my application.

If your variable powered scope has an illuminated center dot consider buying a mount that places the scope centerline at 1.5" if you want to co-witness on 1x or 1.5x. As already mentioned, higher magnification makes the post disappear, at most leave a small ghost like shadow in the very bottom of the scope.