The fish store guy mislead your husband. A 75 gallon or larger tank is the perfect starter tank. I'm sure you've learned by now that a small saltwater tank is very difficult to maintain. That said, it can be done.
First of all, do not add saltwater to keep the water level up. Once the tank has been filled with saltwater, only water will evaporate. The salt stays in the tank. Adding more saltwater will cause the salinity of the water keep going up, until it eventually kills everything. Use RO or distilled water to replace evaporated water. This is "topping off the tank". You will need some fresh saltwater to do water changes, which are recommended very frequently since you only have two gallons of water in the tank. It won't take long for it to become polluted. That's why large tanks are easier to maintain, as the higher volume of water takes longer to get nasty. As the old saying goes... the solution to pollution is dilution. Yes, you'll need a small heater to maintain a steady temp in the tank. Some form of filtration is highly recommended, as it will remove some of the nasties, as well as provide aeration and flow. DO NOT OVERFEED this small tank. Feed the fish very small portions, and let the crabs and snails clean up the leftovers... if any. Fish always act hungry, but will do fine with meager feedings.
I find it reckless on the salesman's part to push this tank an occupants on you. You were tricked into buying a tank that only an expert saltwater hobbyist would even think about attempting. I'd ask if I could return it/them. A perfect starter saltwater tank will include everything you need to get the tank up and running... less critters, or course.