Well, cycling with fish will cause the cycle to last longer than it would had you decided to go fishless. Fish produce waste, which produces ammonia. Ammonia is toxic to fish. Once there is ammonia in the tank, bacteria will begin to produce and "eat" the ammonia, producing nitrite (also toxic to fish). The nitrite will then be broken down by bacteria and form nitrate (not toxic to fish in smaller ppms [20 or so]).
When cycling with fish, you are letting their respiration and waste become the ammonia source. Unfortunately because ammonia is harmful to the fish, it will be necessary to do water changes to keep the ammonia levels low (.25 ppm or less), which will keep the tank cycling longer as the ammonia needs to really establish to build up the bacteria to convert it to nitrite, then nitrite to nitrate. In a tank that big, you might have some luck with only 6 mollies, but you will still have to do water changes to keep the ammonia levels low enough, and your cycle will not be complete for probably two months.
The fish store person just wanted the sale, IMO. Running a tank for a day with no ammonia source doesn't do anything for a cycle. Yes, you can cycle with fish but it's harmful to them. The addition of live plants will help because they will absorb some of the ammonia. A fishless cycle starting from scratch with no seed material from an established tank (like filter media or gravel, which already has some of that good bacteria) can take up to 6 weeks.
Did the fish store employee sell you a kit to test your water? If not, nearly everyone on here recommends the API Master Freshwater Test Kit. It tests everything you need to know about your water parameters during a cycle (pH, ammonia, nitrite, nitrate). It's smarter to test the water to find out what's going on than wait until the fish start acting strangely.