Water Changes more often than weekly.
No corals because you will have to use panacur on the tank to rid it of hydroids (which can sting and kill dwarf seahorses) and Panacur makes an aquarium unsafe for corals and many inverts for up to a couple of years. You will also have to be very careful about the inverts that you keep with them. Those that eat primarily algae won't survive in a panacured tank, and others could easily injure or eat your seahorses.
So, if you are ready to turn your Pico into a dedicated dwarf seahorse tank without corals or tankmates, then yes, you could put dwarves in there.
Precautions you need to take are to make sure that the intake on the filter is covered, that there isn't too strong a flow, that water temperatures are kept in the low 70's, that you are prepared to treat the tank with panacur when hydroids appear, and that you are prepared to hatch brine shrimp daily, that you keep out anything that could sting, pinch, or eat the seahorses (eliminates all swimming tankmates, crabs, hydroids, aiptasia, most shrimp, etc.)... also, think about the fact that unless you have a fish-sitter that knows how to hatch brine shrimp, you can't go on vacation.