FCPX takes care of it for you.
Just drag the clips into the timeline. I've done a few here, and it's fine.
FCPX will render the clips that are not the same as the project settings, so they conform to the same settings.
So make sure you have your project set according to your primary material.. to keep render down.
You can mix frame rates. Start your project in the dominant framerate, probably 30fps in your case. You can easily do this by dragging a 30fps clip into the project as the first clip.
By default, FCP will mix the frame rates using the FLOOR algorithm. This requires no rendering, but you may get better results with one of the other algorithms.
- Floor: The default setting. Final Cut Pro truncates down to the nearest integer during its calculation to match the clip’s frame rate to the project’s frame rate.
- Nearest Neighbor: Final Cut Pro rounds to the nearest integer during its calculation to match the clip’s frame rate to the project’s frame rate. The Nearest Neighbor option reduces artifacts at the expense of visual stuttering. Rendering is required.
- Frame Blending: Creates in-between frames by blending individual pixels of neighboring frames. Slow-motion clips created with Frame Blending appear to play back more smoothly than those created with the Floor or Nearest Neighbor setting. This setting provides better reduction of visual stuttering, but you may see some visual artifacts. Rendering is required.
- Optical Flow: A type of frame blending that uses an optical flow algorithm to create new in-between frames. Final Cut Pro analyzes the clip to determine the directional movement of pixels, and then draws portions of the new frames based on the optical flow analysis. Choosing the Optical Flow option results in better reduction of visual stuttering, and Final Cut Pro spends a significant amount of time to fix visual artifacts.
for more see here.... http://help.apple.com/finalcutpro/mac/10.0.5/#ver3363b44e
For simply editing you do not need to asjust anything, just drop stuff into the Timeline. The info given by AppleMan1958 is for "retimeing" and "conforming" clips (another method of retiming) only. For simple edits, you don't need to do anything. FCP X handles it perfectly fine. Always has.
"Everything I read says DON'T MIX FRAME RATES (or mix at your own risk)."
You've been reading stuff from folks who've never actually used FCP X.