Can traditional CMS/eCommerce be used with a headless platform?

This depends on the traditional platform. With a headless CMS, the key difference is its ability to create different content types and then make that content available through an application programming interface (API). 

If a traditional CMS is able to process API requests it can be integrated with a headless platform. If it's not then programmers will need to process the API requests outside of the traditional CMS, through the use of javascript in the display of the website.