Any dwarf angels such as flame, lemonpeel, cherub pygmy would be nice in a 40 gallon and they probably won't go after your clownies. Any gobies would be a great addition to a small tank too. I would suggest highfin shrimp goby and the clown goby. Both are awesome fish and the shrimp goby may pair up with a pistol shrimp.:happyfish :happyfish