I am here to comment because Stephen emailed me about this thread, sorry it took me so long to get here but I have been out of town for a couple of days.
First thing I want to say is that I disagree with a lot of what has been said here. 1 tbsp of salt for each 10 gallons of water isn't even going to register on a hydrometer. Many molly species, such as the silver mollys can handle full saltwater conditions, with spg/salinity of 1.023 - 1.025.
I have heard no evidence here that the salt caused any of the problems in this tank.
If salt/salinity were the problem, the first fish to be affected by it would be the angelfish and paradise fish who are very sensitive to salt.
I did catch that there was a period of cloudy water. That tells me that there were some fluctuations in the water params, and that does not include salinity. Adding 6 new fish to a 40 gallon tank that is already fully stocked will affect ammonia levels, which I noticed were not listed. Once ammonia breaks down, that means a spike in nitrite. Quite often when the cloudy water hits, it's during a nitrite spike.
There could be any of a number of things that caused the deaths of the mollys, but considering it was just the mollys that died and nothing else has been affected, I would have to think that there was something specific with the mollys that killed them all. It could have been a bacterial infection, fungal infection (early stages don't always show symptoms) the stress of movng them or even a change in their water params from store tank to home tank. With the information that was given, there is no way to say for sure what it was that caused the deaths, but I can say with confidence that I really don't see anything to indicate the salt caused this.
It would be a good idea to keep an eye on the other fish at this point. Not knowing for sure what the problem was means that it could be something that was contageous, especially to other livebearers.
I have been raising mollys for many years. The conditions have varried from saltwater to brackish water to freshwater. Temps have ranged from 75 - 82, and conditions have been very soft to very hard... but never have I had any problems with any of those situations. Mollys are huge carriers of fungal and bacterial diseases, and that is the #1 problem found with them, overall. They are susceptible to a lot of things because there is a large amount of inbreeding happening due to their breeding habits. Inbreeding weakens the immune system, which then makes them more prone to illnesses. I can say that after many years of study and experimentation, I have found that salt only contributes to healthier and stronger mollys. Silver mollys kept/raised/bred in brackish to full saltwater tend to be twice the size of mollys kept/raised/bred in freshwater. Salt content strenghtens their immune systems, and disease/illness problems lessen when conditions are brackish to full salt.
Stephen, you might want to check with the lfs where you purchased those mollys, ask them how much salt if any is in their molly tanks. Going from brackish to freshwater can be quite stressful on them. Your tank, even with the small amount of salt you put in there, would still be viewed as freshwater, and the thriving angelfish and paradise fish would be proof of that. Losing a large amount of salinity would cause problems such as you described, especially if it was all at once.
One last note... feeder guppies are quite tolerant of high salinity levels, fancy guppys and swordtails are not. Feeder guppys can be dropped into full saltwater conditions without acclimation and still survive. Anyone with a saltwater tank using feeder guppys for a food source for carnivore animals can testify to their sturdieness, and that is one thing that makes them so popular as saltwater feeder fish.