Final Cut Pro X to After Effects Workflow
It is certainly possible to use FCPX and AfterEffects as part of the same workflow.  To do this effectively, you will need to use an intermediate codec that preserves image quality across successive generations.  ProRes HQ 422 is a good baseline as an intermediate codec.  There are higher quality ProRes codecs (XQ 4444) and lower quality ones (ProRes LT 422).  But ProRes HQ should be a good starting place.