Clown gobies and blennys will get along just fine.
You will need a mature tank for the nem as mentioned and you will need strong lighting...something to conisder...and also, not all clowns will host a nem, it's hit or miss with them.
So don't get the pair if you are really really wanting to see that cause it may not happen.
As far as coral, it will depend on your lighting. What are you going with or do you have already?