Thank you for your post.
Having everything in one file will make things quicker, as everything is referenced in one place. However, some users may use a customer table with a different application, so having it as a separate file may be useful. The speed difference is negligible.
The database file size is limited by the operating system. If you plan to have a database file that is going to exceed 8 terabytes, then you may want to split the tables into separate files.