Be very careful adding salt to the tank with a betta, some can be very sensitive to it. In addition adding freshwater aquarium salt to a tank does not make it a brackish tank. Brackish tanks use marine salt and usually have a salinity of ~1.015. In addition depending on the attitude of your particular betta, you may find him picking on the mollies.

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29 Gallon Planted
1 Angelfish; 2 German Blue Rams; 11 Rasbora Boraptensis; 3 Khuli Loaches, 1 BN Pleco

20 Gallon Planted
2 German Blue Rams; 5 Corydoras Paleatus; 4 Otocinclus Catfish; Amano Shrimp

5.5 Gallon
Betta, Some plants.