Your needs sound simple but are complex to do and manage them to work properly without conflicting with the existing core functionalities.
You can actually add/modify/delete any field type from the content manager since its code is extensible through extensions. I would recommend looking inside the strapi-plugin-content-manager source code and see how the default fields are created and how their custom attributes are added to the admin.
But please remember that it will be hard to maintain future updates if you use extensions.