Hello.

The betta could be killing off the mollies. Bettas are aggressive fish, and shouldn't be kept with mollies (community fish). The betta may or may not be nipping at them, he could even be chasing them and stressing them out and then they die from stress.
The soapy film is not anything to worry about, but still try to keep the surface clean. I have had this problem to and had healthy fish. (I currently have no fish because my 2 bettas died of gill disease, and I am waiting for a bigger place so I can have a 55 gallon with a Jack Dempsey).
If your one molly had dropsy, that is a symptom of a tank with bad water quality (although it is possible to have a fish with dropsy without bad water quality). Nitrates really wouldn't kill your fish off so fast, but ammonia would, and you said it's fine. Anything above 0 is bad. Ammonia is lethal. You should be changing your water 10-20% biweekly.

Once your mollies died (as well as any dead fish), they do have white "fluff" on them. That is natural, and nothing to worry about although as soon as you know your fish is dead you should remove it.

As for adding more fish I would wait a little while before adding any more. Have you checked places like liveaquaria.com for replacement filter cartridges?
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Phishie

"An ounce of prevention is worth a pound of cure."

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