So, here is what I understand about mollies in particular as pertaining to the type of water they like.
Most brackish water fish are just saltwater fish that have evolved to deal with a lower salinity. True brackish water is very close to having the minimal salinity needed for a SW tank. This happens because the fish have been trapped over a long time and landlocked into ponds/rivers/lakes that raise and lower in salinity when it rains or when the water evaporates. (This is obviously not the whole story, but a good example for at least a little bit of understanding of the concept).
Mollies are fish that can literally be kept in any type of water provided that they are acclimated over time to one or the other. For BEST results, like a molly obtaining a size of 4 to 6 inches, I believe brackish water is needed. In freshwater, they are not really going to die sooner or live longer but they may not be as big or as colorful as they could be. Most of the 'special' types of mollies (balloon, lyretail, sailfin, etc...) have been bred this way in brackish water and so I think brackish water tends to be the best for them, IMO. Still, you shouldnt be having all too many problems with mollies being overly mean and killing off tankmates.
I think the problem you are having is that both mollies are of the same sex. I dont think it would matter if they are both male or female, since they both want to be dominant, breed, and do what fish do I guess. I think that if you had one male and one female, you are going to have a 10 gallon tankful of fry in a matter of weeks. In such a small tank, it may be best to keep only one molly and one livebearer for that matter.
I still dont think that the tank would be completely full though and you could get some other fish and maybe a school of smaller fish like neons. If you have enough of them (6 to 8), they can take care of themselves. It may also help to keep the smaller fish in the tank for a couple of days before adding a molly or other large fish. I also think that a bottom feeder type fish would be a good addition, BUT a pleco may be something to avoid since even the smallest varieties are going to be big waste producers. I like the cory idea myself and I am not really a cory fan (sorry, cory lover! Just so you know, I dont hate them either)
The betta fish is going to have problems with being in a tank with other fish who have big fins and so a molly may not be the best choice to mix with a betta. Bettas, in my opinion, are also best kept either alone or with fish that are not overly active and schooling fish who could 'gang up' on it.
I think you should research out the fish that appeal to you the most in certain aspects. I dont think that you seem like a person who wants a tank with one fish in it so look into others. Even though you cannot get bunches and bunches of fish, a small school would certainly fit, if the fish were small like neon tetras, rummy nose tetras (my personal fav.), glo-light tetras, danios (not giant, just normal), or even the smallest fish on my 'most wanted list', the glow-fish (check it out)!
Just dont get too discouraged by your recent problems with the mollies. I think that they are often times not given proper credit or emphasis on their teritorialness (if that is a word) and they get a lot larger (and hence stronger) than some realize. In a livebearer tank I used to have up and running for breeding (mollies, platties, guppies mainly), I had one very very very large female who ran the tank and would even attack my python when I tried cleaning the tank! There is hope though and it could just be that you found yourself a molly with a particularily grumpy disposition!