Can you keep freshwater fish in Brackish tank??

Depends. There are some freshwater fish that can tolerate mild brackish conditions. Mollies can actually go the full range of marine to fresh, but prefer slightly harder water.

Do you have specific fish in mind?

Cories won't go to brackish. They can tolerate medicinal levels nicely, but not brackish.

I have to say, keeping fish in water that is seriously different than their native water is pretty well asking for trouble. It tends to stress the fish, increasing their susceptibility to diseases and parasites. They also are not as colorful and active when stressed.

I've never kept danios in brackish conditions, so have no idea.

Converting is just a matter of using brackish water for water changes. Start with a low level of salt, less than 1.005, and gradually increase the amount of salt with each water change.

You will have to find a new home for the cory, and I would remove the danios as well. Just not a good idea to stress fish.

Maintaining the tank will be a bit more work. You have to test for salinity, and preparing the water for changes is an additional step. Cleaning is pretty similar to FW--though you have to deal with salt creep. FW topoffs for evaporation are critical in BW, since the evaporation increases salinity, and can cause problems.

Otherwise, it's not that different to maintain. You will be limited in what plants can go in, as well as fish. There are many, many fish sold as brackish that need full marine conditions as adults--and compared to either FW or SW, there are not that many BW fish in the trade.

Don't know about dwarf livebeaers but all the others do fine in freshwater and are most likely kept in freshwater at your fish store. If you really want a brackish tank I would suggest either getting a new tank or else give up your other freshwater fish. It will just be easier that way.

Also don't get salt for freshwater aquariums. It is just NaCl. You really need to buy marine salt.