I agree if you put them in at the same time you have a good chance....but not after the clowns have established themselves. My trio tried to kill the Naked Clown pair I put in.....and they had not been in the tank for a long time...just long enough...they're in a 150g and they use the entire tank. When people say clowns don't swim around much...well mine do...one side of the tank to the other...and they play host with ... a wavebox...no corals I have offered or anything. They did start to play in a large brain coral I had...but then I had a fish nipping it so the brain came out..and back to the wavebox.
GSP I totally get the wanting the Grade A or Premium pair. They are much nicer and great colors...now if only we could get some Snowflake or Snowcasso's in.....THAT would be cool! Here is a thread that sort of gives an overview of what this guy grades his clowns as;
http://www.donisreef.com/examples.html