When I add salt to my livebearer tank it is at the rate of 2Tablespoons per 5g of water. So, in a 40g tank, if starting from ground zero, add 8 Tbsp. of salt. It should be Salt for reef tanks and not aquarium salt. Aquarium salt is for medicinal use and adds many micro elements to the water and raises salinity only slightly. I actually raises hardness more. I do not use reef salt. Ad the tanks are NOT brackish. And if you are wanting to go brackish, as suggested, I would get a hygrometer or a salinity difractometer.
Now, looking at the mix of fish in the tank, one cannot have a brackish tank with fish such as paradise fish and tetras. This fish will not tolerate such conditions. So, a family tank of livebearers would be best wit hnone of the other fish in their tank.
Temp may also be high for the livebearers. I keep mine around 72-74 degrees. I word of caution here. At such low temps, the possibility of ich increases. Keep an eye out for any outbreak.
So I guess that my suggestion is to keep the livebearers in a tank of their own. Get the equipment suggested. Another thing that I might do is a 50% water change 2-3 times over the next 7-10 days. I would then measure the tanks salinity and add sea salt accordingly as I did my regular water changes. Increase the salinity gradually.