I haven’t seen the warning you are getting, and I can run SketchUp/Layout just fine on macOS 10.13.3 High Sierra. Do you have the last version (17.3.116) of SketchUp 2017 installed? Check SketchUp->About SketchUp to see. That was an unusual off-cycle release specifically to fix some serious issues introduced by High Sierra. There were a few remaining fringe issues, but on the whole 2017 will run.
But this has nothing to do with installing SketchUp 2018, which is fully compatible with High Sierra.
Or are you perhaps referring to the warning that your file was created by an older version and will no longer be readable after opening in 2018? That warning could perhaps be worded better. It really means that the 2018 file format is not readable by older versions and you need to do “Save As”, not just “Save”, and choose the older format from the selection list at the bottom of the Save As dialog. Yes, you have to do that each time you save the file unless you are ready to simply move ahead to 2018.
I don’t use section cut face plugin, so I can’t say for sure whether it works in 2018, but there were small enough changes from 2017 to 2018 that the majority of plugins will still work.