They are saying not to add instant ocean (marine salt) becuase of your fresh water fish which is correct because freshwater fish can't live in Brackish water. To make Brackish water necessary for your crabs you need marine salt and instant ocean will fit the bill nicely.
iliveinazoo is quite right here. Whether you use tonic/aquarium salt or marine salt mix, if you add enough salt to make the Fiddlers healthy in the long run, you'll make the freshwater fish sick or dead. There's nothing magical about tonic/aquarium salt; it's just that at the "teaspoon per gallon" levels the store likely recommends, the resulting water won't be salty enough to stress freshwater fish. But neither will it be salty enough to be brackish water, and consequently not salty enough for the long-term care of brackish water animals like Fiddlers.
There are some freshwater fish that can live in low-end brackish water (SG 1.003-1.005) perfectly well including Guppies and Mollies, as well as numerous brackish water species not always sold as such, like Knight Gobies and Violet Gobies, so you may want to look at these for stocking the Fiddler Crab aquarium (though some, like Knight Gobies, will be too predatory to keep with small fish like Guppies).
Thanks for explaining even more, I agree, since I can't start a new aquarium right now I will get some gobies, and the mix of salts I understand. When I get back from my diagnostic test today I have to clean entire tank: water is cloudy white. I also have to get some screen for the top (stores don't sell tank lids with screen) I also have to learn how to read darn hydrometer and check pH. I also don't know why my tank is cloudy! I've done partial water changes, added dechlorinated water, some ammonia, removed most of goldfish (when I had 8 the water was not cloudy), have nre filter (will check on the height of it), have air pump, just can't figure this out! I did not add that much salt that water should be this cloudy, any ideas to fix it?
Cheers, Neale