The plugs that are used in the peepholes are not put in to prevent heat loss, but to stop air from entering the kiln.
There are 3 common ways to fire an electric kiln.
1. By manually turning the kiln on and up, and watching the cones inside the kiln through a peephole to determine when to turn the kiln off.
2. By manually turning the kiln on and up, and using jr cones in a kiln sitter to turn off the kiln when it reaches temperature.
3. By programming an electronic controller to turn the kiln on, up, and off when appropriate.
The worst thing you can do is fire low fire clay or glazes at high fire. The clay and/or glaze will melt all over your kiln and can cause major damage! If you have both low fire and high fire materials in your studio, you might want to mark your pieces differently on the bottom so you don't forget and get them mixed up!