Unfortunately, I don't believe any anemones would do well in a 15g high tank. Aside from the fact that most host anemones get fairly large, trying to create enough light while dealing with the issues that come with it (heat and resulting evaporation) would probably make it difficult to keep the tank parameters stable. I think this would be one animal that would be best to pass on until your next tank
(No one stops at one!)
I also would like to caution you on the clown/gramma combo. I currently have this in my 30g and the gramma has become more and more aggressive in recent months towards the clowns (and flairs at the bangaii cardinal) who were introduced first. There is plenty of territory and food so I cannot figure out any direct cause for the aggression. There is no concrete proof, but I wanted to relay my experience. Perhaps a more sedentary goby would make a more peaceful addition.