I would not return to that store for information. They are way behind the times about the nitrogen cycle, stocking and water changes.
Stocking: The 1" per gallon guide works ONLY for fish under 2" long, and says nothing about the social needs of the fish at all. Look up each fish you are interested in. Make a chart that includes the optimum water chemistry (soft water, hard water, moderate) and temperature (low, mid or high 70sF or 20sC).
Start eliminating fish that do not fall in the same areas of the chart. In a tank your size I would not keep any fish that get longer than 3", and then only slow moving fish that size. Much better to keep smaller fish so their activity level suits the tank size. This small a tank is too small for territorial fish like Gouramis (maybe one Honey or Dwarf, but not 2 or more). Look into the social needs of the fish.
Mollies are hard water fish, not compatible with the other fish in this thread so far.
Water changes: Until you get really good at knowing what is going on in the tank DO water changes. You do not need to do large ones, but try simple ones like 10% to get to know how to use the equipment. When you get the test kit you can decide about frequency and volume.
It used to be thought that certain bacteria were the ones in the tank that removed ammonia and nitrite. They were wrong. But the aquarium companies used the wrong species of bacteria because they were easy to deal with, they entered a dormant phase that made shipping and handling easy. They could even be dried out and would come back to life.
Among other things, these dried bacteria were glued onto certain filter media and you were told the tank would cycle right away because these bacteria were already on the media.
Unfortunately those bacteria are the ones that live for a few days, a couple of weeks, then die off. The real work horses of the nitrogen cycle are slow growing, and do not enter a dormant phase, so they are hard to package, hard to ship, and will not live if they are dried out.
What I would do now:
1) Order one of the products with the real nitrifying bacteria. Look for Nitrospira on the label. Some products with the right species:
Tetra Safe Start
Dr. Tim's One and Only
Microbe Lift's Nite Out II
There may be others, but do not waste your money on something anyone says '...is just as good...'.
If you need to order it on line pay for fast shipping.
Also, get a test kit. You will need ammonia, nitrite and nitrate for sure.
2) Daily 10% water changes until you get a test kit.
Use the test kit to determine how large and how frequent water changes should be.
3) Research the fish: Running a heater is usually necessary for tropical fish, but it sounds like you have it way up there. Harlequin Rasboras originated in water that is not really too warm, and will do just fine in a tank at 22-25*C.