First of all, you don’t need to have a pair of Clownfish...a single Clown will be fine! I have two nano tanks with a pair of designer Clowns in the one tank and a single True Percula in my other tank.
Here is the problem I foresee with adding a pair of Clowns to your tank.
You may have an issue with your Firefish and/or Royal Gramma becoming scared or picked on by your potential newcomer/s. Clownfish are territorial! A mated pair would pose a bigger problem due to elevated aggression in their territory. A single Clown, not so much.
As for CUC, you may want to try adding a Black Diadema Sea Urchin and increase the amount of snails and species to what you currently have. Also, up the number of hermits too. Caribbean Red Legs and Scarlet Reef hermits are an ideal choice. Look into adding shrimp and a couple Emerald Crabs to your population as well.
Thank you! The Royal Gramma already bullies the Firefish. I had a couple trochus snails but they died in a few days. My hermit is a jerk, it killed one of my nessarius, so I am kind of anti-crab.
What kind of shrimp do you suggest?

