I don’t know if this will specifically answer your question, but when you add a 1080 clip to a 720 timeline, by default, the spatial conform is set to fit. You’ll need to change that setting, highlight the 1080 clip, in the Inspector under the video tab twirl down Spatial Conform and set that to none. What FCPX doesn’t do is auto scale your clip to 66.6% by default it leaves the clip set at 100%, however, you now can scale that Clip between 67% and 100%
Hope that helps!