Versions of Photoshop have different installation trees and user preferences. You do not merge Photoshop versions. Each version is independent. When you install newer versions of Photoshop the are options to migrate your presets and Preferences. You can start and use any version installed on your machine. For the most part newer version of Photoshop are backwards compatible with older versions of Photoshop. However some changes Adobe make to Photoshop change user interfaces and and the way Photoshop performs some functions these changes break some user actions, scripts and user work-flows. In fact all the version you listed CS4, CS5 and CS6 have comparability issues. In that respect Photoshop CC is not any better it has compatibility issues.
Newer versions of Photoshop have new functions and updated old function. Older versions can not handle everything save by newer versions of Photoshop. If you save PSD and PSB files with the maximum compatibility option checked. Older version should be able to open you layer document with layers that look like what was saved though some layer may be altered and may even be a different layer kind then was saved. If you need to support older versions of Photoshop always use the maximum compatibility option.