There are many arguments as to what constitutes brackish water. In the wild it can vary enormously from almost totally fresh to almost completely sea water. the problem is further added to by the fact that many brackish species spend much of their time moving from one extreme to the other! In the case of mollies I wouldn't worry too much about having very brackish conditions. 1tbsp per 5 gallons should be fine for their natural lives. Aquarium salt is usualy fine for this and also for many juvebille brackish species as I have found that too much sea salt sends the Ph over 8 which can be too high for thoe species found in fresher waters.