It's best to prepare your saltwater in advance. It really should be aereated for about 24 hours before use to allow for balancing. If you have the ability to make your water (barrel, trash can etc.) separate then you could arrange your rock in the tank and then add water. It's easier to aquascape without water. If you don't have that option then putting fresh water in your tank and adding salt and then curing should take place before putting your rock in .
As for a snowflake and a fish. Snowflakes feed on crustaceans in the wild and will not bother your fish.
Good Luck!