Choose woodworking, it’s in mm.
You can customize that template to your liking, set your favorit styles and make scenes for default views you want.
If it’s like you want, save as template and choose to make it your default template.
Next time you start Sketchup it will be with your own template. And you can always change it and make a new one again.
3D printing template has a printvolume wich you can ajust to your printers printvolume. It’s basicly useless.
If you do actually want to 3D print, just draw your print volume from the model origin, make it a component and lock it. Or just take care with the size of your model.
Generally, for small complex objects to be 3D printed, just use metres for mm or even for ( decimal) inches.
Scale appropriately in your slicer after exporting as STL, or scale down in SU before exporting as STL.