Mac OS compatibility with PS6

And to make sure: Are you talking about Photoshop 6 or Photoshop CS6? 

When one decides to stick with obsolete software one should make sure one is able to roll back any and all software and hardware updates in case they »break« the set-up. 

See the official system requirements at the following link

https://helpx.adobe.com/creative-suite/kb/cs6-system-requirements.html

People do report it working fine in El Capitan etc as well but it's your luck, not officially recommended. Read the following discussion as well, it will help you take a decision

https://community.adobe.com/t5/get-started/compatibility-of-cs6-version-apps-with-mac-os/td-p/10271326

-Manan