Anything you hear at a pet store should be researched... Mollies like a lot of swimming room which the 10gallon does not provide. A lot of fish stores just want to make a sale. If the fish is suppose to live for 5 years in a proper environment they don't make much money. If they sell that same fish to a smaller tank the fish will get stunted, stressed, and pass away a lot sooner then it's suppose. This way the store sells more fish. The right store would give you an Oscar for your tank. Petsmart trains their employees for selective selling so they can push the product onto the customer.
Now I'm not saying you did anything wrong. It's not your fault. If you research the fish that you have you can find out more about them like schooling sizes, tank sizes, and water param.
As for the babies if you don't want them then don't worry about them. The will just be fish food and if not you can take them to your lfs
and get something for them or a store credit maybe.