As long as the fish from your community tank are well-established fish that you know are healthy, then I think it should be safe to house them in your vacant seahorse setup temporarily while you why are preparing for the new tank.
But it’s never a good idea for you to use your seahorse tank, even if it’s currently vacant, as a quarantine tank for new fish. A situation like that would be unacceptable and present a definite risk to the seahorses that will eventually be living in the aquarium…
After you remove the fish from the seahorse setup and transfer them into the newly established aquarium, you can always empty the seahorse tank and then run it with freshwater for several days as a precaution before you refill it with new saltwater…
Good luck with your aquarium upgrade for your community setup, sir!
Pete Giwojna, Ocean Rider Tech Support