Aside from the cost, a 5.5 gallon reef tank would be a nightmare regarding stability. Many SW critters are very sensitive to slight imbalances in water chemistry, and a single dead critter would pollute the water very quickly.
A small school of neons or one of the tiny rasbora species would fit into a 5.5, and be compatible with a dwarf crayfish if you really don't want shrimp. Alternatively, single dwarf puffer would also work, though it would be a good idea to allow a good number of snails to become established before adding the fish. I recently put a DP into my 10 gallon shrimp/dwarf crayfish tank to control the snail population. The ramshorns have run amok, so there's no danger of the puffer running out of its favorite food. It ignores the shrimp and crayfish.