Can you use CDC on a subscriber for transactional replicaiton
You do not mention whether you are using SQL Server 2012 or 2014, however the documentation on CDC and Change Tracking do not show much difference.

Both make the following statement: "CDC configuration is always performed on the current or intended primary replica." 

http://msdn.microsoft.com/en-us/library/hh403414(v=sql.120).aspx#CDC

The above link is for SQL Server 2014 and offers many more details.