Yeah, your shop guy was very wrong. Most of us do one water change a week. I do two a week in my largest, daily water changes in my smallest, and once a week in my ten gallon.
If you can move out all the fish you've got, you'll want to first properly cycle your filter. Either of the bottom two links in my sig will help you with that.
Later, you'll be able to add guppies (males only, as females will overstock you because of babies), endlers, a betta, pygmy corydoras, dwarf puffers, least killifish, microrasboras, and the like. You'll want to take into consideration that most schooling fish are not appropriate for this size tank because having a full school will overstock the tank. However, pygmy cories and microrasboras are small enough that you can have a few in a tank this size.