If you are installing them in an electric powered plane you don't need the grommets. The main purpose of servo grommets are to isolate the servo mechanism from the vibrations caused by the engine. With electrics these vibrations aren't an issue. Mount them with a screw. Many instruction manuals for park flyer size planes will also instruct that you can glue the servos in place with CA adhesive. However, I myself don't like that option.

Ken