RAM is best purchased in matched pairs at time of purchase. While you may not have any issues, subtle timing issues can crop up, resulting in errors. Running dual channel mode and with XMP only compounds the potential problems with mismatched sticks.
Many things have to be considered, you should add memory by playing safe. Any motherboard will only be compatible with a single type of DDR memory so all you need to do is buy the type that works in your computer. Also you can just buy the fastest memory available but be sure that what kind of memory is exactly used by computer