I would preform a water change while you get your water tested, I agree bad advice the the pet store, don't no if you can keep females with male siamese fighters, i would ask in the betta section of the forum, good luck.
You can keep Male fighters with females, just not two males.
Black widow, are you sure of this information? This goes against everything I have ever heard about bettas. Betta breeders only bring males and females in contact for a very brief time, after they have been carefully conditioned.
Now, red pleco, as to the rest of the list:
5 neons
1 golden nugget pleco
1 red spotted pleco
1 male siamese fighter
4 females siamese fighters
2 plattys
1 white molly
1 black molly
I cannot remember offhand how big these particular two plecos grow; you need to google this. If they grow longer than c. 4 inches, then they are not for this tank. Also, plecos are HUGE waste producers, the last thing you want at this stage.
Neons are not strong enough to go in a new tank; they should only be added once the tank has been up and running for several months.
If I were you, I would get in touch with the shop and point out, politely but firmly, that they have mis-sold you these fish and ask them to take them back.
I would get the male betta his own little tank (but ask for advice on betta forum first).
I would probably hand the female bettas back too, but ask on betta forum for suggestions.
Then I would cycle the tank with the platys and mollies; this should be enough and no more fish should be added for the next 4-6 weeks until your daily water tests show that the ammonia and nitrites have disappeared. Then you can think again about what fish you want and have time to research what is suitable in your tank.
Do not take the shop's advice. They want to sell you more fish. And then in a week's time, when your fish have died, they want to sell you some more.