Sorry, I know conflicting advise is frustrating. But I have to say that Mumma is right. Not all petstore employees know their info.
There are a couple of potential issues here. First of all, since the tank wasn't cycled, ammonia builds up and is toxic to the fish. You can fix that though with water changes. It was bad advise to tell you not to change the water. Did you read the cycling links? They are helpful.
Another issue is that honestly 10g is a bit small for mollies. They get kind of big for a 10g tank. But, since they are probably still young, that isn't necessarily the issue at hand right now.
Lastly, not all stores treat their fish well, so you have gotten fish that were a bit unhealthy to begin with accidently. That coupled with the ammonia is a lot for a fish.
No worries though, do a big water change and see if the fish perk up a bit.