I catch blue crabs up in Washington, DC on the Potomac River in the summer where salinity is pretty much 0. I have kept small ones in freshwater aquariums where I had only a small amount of aquarium salt and was keeping normal freshwater fish species. So if given some time to adjust from a saltier home to a freshwater home they would most likely survive OK. Since you live in Louisiana, they probably won't have issues with winter kill.
As far as how they would affect the balance of the pond? Not sure... They may prey on the eggs of all species some, but I don't think they would be overly predacious on fish eggs. Not sure they would get enough food in a small body of water like yours to have reasonable growth rates though unless you supplementally fed them.
