Yes, sir, that should be a workable setup for a dwarf seahorse tank.
The Nassarius snails love to bury themselves in the sand, but I think you can include one of them in your dwarf tank with a bare glass bottom without any difficulties. The Nassarius snail should quickly adapt to the glass substrate.
No, sir, even with a low airflow, just one of the sponge filters is all you should need for a five-gallon aquarium. It will provide all the biological filtration you need once the aquarium has cycled and two of them would occupy an awful lot of space in such a small tank.
In short, I think you have outlined a good plan for a dwarf seahorse tank, Richard, so all you have to do now is get really good at decapsulating brine shrimp eggs and hatching them out in large quantities on a daily basis.
Pete Giwojna, Ocean Rider Tech Support