Metal plugs for my house address set are missing. I believe the metal plugs are what the screws go into in the wall. If I find plastic wall plugs that fit the screws do you think that would work?

No any extra wall plugs required for the installation. Your installer needs to drill holes in a wall and push the letters with metal screws inside the holes, that’s it. Metal screws are going inside the holes without any extra plugs. See the attached image please.