Proper way to play 50 fps video as 25 fps in FCPX
Your project's frame rate should be the playback frame rate.  So if you want to output 25fps, make it a 25 fps project.  When you import other frame rates, many NLEs will attempt to automatically adapt the frame rate to your project rate.  If FCPX does not do this for you automatically, then you can manually set the timing of the clip to play back at 50%.

But when you make a 50fps project, you are basically making it impossible for FCPX to do the right thing.  Setting a clip to play back at 25fps in a 50fps project is meaningless.