Working with the .stl file is problematic due to the triangulation of all surfaces. If you must work with it in SketchUp you would find it better to import it with Meters as the import units. Also selecting Merge coplanar faces in the Import options will help. Even then it has problems but they will be easier to manage. Seems to me it would be better to use the stl as a reference and create a new, cleaner model of it to avoid problems like those that have been pointed out already. Maybe it's possible to get a 3D DXF file from the original source? Maybe it would have fewer issues. Hard to say from here.