It all depends on how high you are going to take the SG (specific gravity) of the brackish water, purple spotted/striped gudgeons (Morgurnda species) can tolerate light brackish conditions to a SG of around 1.002 but cannot take the same kind of salt levels required to keep the more common brackish fish like scat's, mono's and morray eels which preffer the SG to be around 1.010.
What other fish was you planning for your brackish set up?