Can I still use my iPhone 6 Plus headphones on my 7 Plus?

Because they both use a Lightning connector, they are interchangeable. You can also connect ANY headphone to your iPhone using the short adapter that was included when you purchased your phone (or by buying an inexpensive replacement from Apple). You can't charge the iPhone at the same time you are using your headphones, but this seems not to be a problem for most people, despite the fears of many when this connector was first introduced.