If you install the free Unity these scripts are in the folder “Assets/Standard Assets/Scripts/Utility Scripts”.
Yes, you can submit them to the Asset Store inside your package. I have done this with one of my tools, but I have renamed these scripts and made some changes to the MeshCombine script to match to my needs.
If you upload these scripts with the original names and someone download it into a project, where these scripts already exists in any folder, he will get errors in the console.
You can choose the package(s) you want to have inside your project, while you create a new project.
Or you can add packages to your project by going to the menu “Assets->Import Package” and choose “Standard Assets (Mobile)” for example.
The combine scripts are in “Scripts.unitypackage”, if I remember right. But not sure at all.