Can you push out the SEP 12.1.6 client to a workstation managed by a SEPM with 12.1.5 installed?
--> Yes, it's possible but Symantec recommends to keep SEPM and SEP clients on the same version.
Actually you should consider to upgrade the SEPM first.
Symantec Endpoint Protection 12.1.6 Release Notes/What’s New
http://www.symantec.com/docs/DOC8626