That fish is going to be too big quickly, even if you get him small enough at first.
Instead, consider fish such as the Barnacle Blenny that are sized for that tank - about an inch long, fully grown.
I love my Barnacle Blennies!
Or consider doing coral instead of fish!
I love my corals!!
I would also leave out the Astrea cuz they get huge - he will starve to death unless...
...those 5 hermit crabs get him first. I would leave out the hermits....or have no more than one and get him in a different size than your snails, or say goodbye snails. (He may still kill your snails just for pleasure. Hint: Hermits wear snail shells. Kinda silence of the lambs, isn't it?? )
I would do a couple nerites and a couple ceriths at most....only if you can get em really small.