You can buy some base rock for cheap to fill out the rest of your rock. It will become live after a few weeks and pretty soon you won't even be able to tell the rock apart! lol
As far as cuc I would add a few snails and hermits, nothing crazy as you don't have much to feed them right now.
I've heard that urchins can knock corals and stuff around so I've stayed away from them, but as mentioned they eat ALOT so you'd want to add that one after a while.
I happen to love shrimp and they make great cleanup too, I also have a serpent starfish, very cool and eats when I feed the tank.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.