Might be a scripting solution but does anyone know how I can attach a single doc to all the customer records in one go? The thought of adding a custom field however would mean having to go into each customer and update. Also, want that anytime a new customer is added, it has the same single doc attached?

you can use csv to set that file on the existing customers. to have it attached in future just set a default value on the field. Heck, if all you really want is a link to the same file no matter what - set it as a non-store field and it will be whatever you set in the default value