The clown definately makes this difficult. Full grown he will be just a tad large for this system and other fish will find it hard to hide/avoid from him adn they can be quite mean. The fish that would work well with him are either too large for your system and/or not reef safe. Perhaps a small dwarf lionfish, although he will spend most of his time perched on rocks, similar to your hawk, and I'm guessing you were wanting something that will stay 'out and about'. A damsel could prolly hold his own, especially if you rearranged the rock pile before introducing it to the tank, and is an appropiate size for your tank. The clown really should have been introduced last, or perhaps not at all. In a tank this size you have a lot more options with less aggressive fish. Also, the clown and the hawk as adults will be a lil large and if you add much more to the system you are going to really have to stay on top of your water changes, and I would recommend adding a skimmer if you haven't already. Also, there a mods online for upgrading the aquapods filtration. Good luck with the new fish.