It may or may not work out. Please understand that the needlefish will do better in a light brackish setup S.G. 1.003 - 1.005 than it would in pure freshwater. The rocket gar will do better in pure freshwater. I understand that while preferring freshwater the rocket gar can handle light salt content.
If I were to try the combo. I would start at a S.G. between 1.002 & 1.003 and ONLY USE MARINE SALT!