Can Photoshop CS6 and Photoshop CS6 Extended files be saved in a format that can be used on CS5?
In general there will be no problem - save Photoshop documents in PSD or PSB format for best results.
There may be a loss of some 3D layer data when moving from CS6 Extended to any other version.
See this thread regarding a difference with Shape layers: Do CS6 files run in CS5/5.5
Retrieving data ...