I understand that when starting a new project the initial clip specification i.e. resolution and frame rate determines the remainder of the project. I normally shoot in 4k @ 30fps with my Z6 and GoPro with the latter. I have just bought a drone and would like to shoot 4k @ 60fps to allow slow motion. If I start the project with 4k @ 30 fps – for instance – then import 4k @ 60fps FCP shows this footage at 30 fps. Does this mean FCP is dropping the frame rate to 30fps and what happens to my slow motion effect ?
Automatic Speed doesn't require any particular multiple or rate. What it does is play every frame in the clip at your project's frame rate.
For example: you're editing in a 30fps timeline and drop in a 1-second 24p clip. If you leave it alone, FCP will add (duplicate) a few frames so that your 1-second clip still takes a full second to play back in the 30fps timeline.
Now select that clip, and choose Automatic from the retime menu. The clip will be sped up slightly, because you've got 24 frames playing at 30 per second.