Welcome to the fcp.co forums NB
FCP is Mac only so if you need to preview using an iPad/phone it is necessary to use a different app that will export to FCP.
iMovie is available on iPad/phone and has the ability to export to FCP. Free and when you import the media in, it will be conformed to Apple's codec/format so iMovie and FCP can share resources.
Another approach is LumaFusion by LumaTouch. It is a 3rd party editing app that you can use in place of FCP or you can export via FCPMXL (requires add on). Lots of folks like LumaFusion.
Personally I edit on a laptop and import directly into FCP so don't use the above approaches. I import media into FCP on an external drive, and use quick "I & o" plus tags to create my rough clips. Very fast and the laptop serves as my mobile device, but I get wanting to use an iPad.
Hope this Helps, Happy Editing, Greg
I have used LumaFusion to make rough edits and export to FCP via FCPXML. Looking at the LF manual it does not appear that Timeline Markers are exported. If you try it you will want to have the media on an external SSD that is connected to the iPad.