There are two different versions that are common for both the 5C and 6X heads. There is the 5C-4 and 5C-8 and 6X-4 and 6X-8. If both heads have the same secondary stamping (4 or 8) then they are identical for all practical purposes and no problem running them together. Here's how you find the secondary stamping:
The #11 heads will probably offer a better compression ratio due to smaller chambers. For performance work they will need to be converted to screw in rocker studs and larger valves. The ports will also need minor work to flow the same as the larger valve heads. Any good shop with a modern Serdi or Sunnen head machine can do this work for you without a whole lot of extra cost.
Hope that helps.