Things to take into consideration when stocking such livebearers are;
a. Swordtails need at least 2-3females per male, however no more than one male can be kept per tank as unlike other livebearers like mollys and guppys etc, males will fight each other all the time unless there is a very large number of females per male (something like 15 or more females per male etc).
b. Swordtails can crossbreed with platys very easily, they are very similar fish. Personally if i were you, i would either opt for swordtails or platys but not both.
c. Platys, mollys and guppys all need at least 2-3females per male, you can have more than one male per tank as long as you have enough females per male. On the other hand though, all of these common livebearers (including swordtails) are notorious breeders, with females having the ability to give birth to over 30 fry every 3-5weeks. So the more females you opt for, the more fry you will have to deal with, and the problem many livebearer keepers face is simply what to do with all the fry. There are many options when it comes to dealing with large ammounts of fry though.
d. Mollys do prefer slightly brackish conditions. There are many varieties of mollys, some of which are more freshwater or brackish than others, however most varieties of mollys do best in slightly brackish conditions. Platys, swordtails and guppys can be kept in slightly brackish conditions, however turning your tank slightly brackish will limit your stocking options a lot when it comes to freshwater fish, since there are a lot of freshwater fish which cannot tolerate salt well and it will have a negative impact on their health, even kill them.
e. These livebearers will primarily occupy the middle to upper range levels of the tank, the problem with all-livebearer tanks like this is that it will leave the bottom levels of the tank very empty and inactive looking. However a lot of bottom dwelling fish in the hobby are intolerant of salt, so if you wanted mollys in a slightly brackish tank, a lot of bottom dwelling fish like numerous types of loaches, plecos, corys etc would not be able to be kept safely and healthily in the tank in the long term.
So a lot when it comes to keeping mollys basically depends on how much you want to keep certain other types of fish in the tank, and whether you want to sacrifice having those certain other fish so you can keep certain types of brackish loving mollys etc- there are still quite a few brackish loving bottom dwelling fish though available, although not as many as freshwater ones in what i have seen so far my experience.
How many of these livebearers you could fit in a 65gal tank depends on how many you want roughly of each (i.e. for example "i want guppys, but more swordtails than guppys", or "i want mainly platys and guppys" etc).
What are the exact measurements/dimensions of the tank? The exact measurements of the tank are very important when it comes to deciding how many fish you can have and what are suitable for the tank etc.