As others have said, danios cannot be kept in brackish water.
If you need hard, alkaline water for livebearers, then the schooling fishes to investigate are things like (certain) killifish, medakas, halfbeaks, and (most) rainbowfish/atherines. At least some of these should be easy enough to obtain through your local fish store. Celebes rainbowfish, Telmatherina ladigesi, are colourful and small and lively, and should do very nicely alongside most livebearers. They'll replace danios nicely. They tend to be a little more expensive than danios, but not much, and like rainbows generally, compensate for price by being durable and long-lived.
Most of these fish will do well in slightly brackish water, too, but you'll need to do a little research just to be sure. The Brackish FAQ I edit (see below) is one place you might start.
Cheers,
Neale