I have a 25g tank with CC. It has a small stripped damsel, two clowns and a purple psudo. The fish share the tank with a condy anemone, chocolate chip starfish, an electric scallop, an anemone crab, a couple snails and a couple hermits. The only fighting is between the psudo and damsel and they usually respect each other's space. I had a purple pincushon sea urchin but it died a few months ago. (either water condition or improper feeding, shame on me)
So you can have fish and inverts in that small of tank.
The inverts are soooo cool.
Good luck
bad