betta fish
Fish Herder
I think your tank size is fine for what you want, I don't know much about Hawkfish but have read they do clash with CUC as that is their diet in the wild although there are different types of Hawkfish and some are more peaceful than others so it depends what type you want, check on Google and see if any are compatible, Goby and pistol shrimp pairings are awesome, I have a pair, they have a symbiotic relationship in where the goby is the pistols eyes as they have poor eyesight so if the goby is out of the tunnel the pistol knows it is safe to come out too if the goby dashes in so does the pistol, the pistol benefits the goby in that it builds and maintains the cave they hide in, my goby also feeds the pistol