Your tank is seriously overstocked and you'll probably continue to have problems until you remedy this. I can't imagine how the pet shop can say everything is fine, your parameters should be all over the place with the bioload. I would strongly suggest you get some tests kits to use yourself. Peek over the salesguy's shoulder the next time he tests your water -- have him explain the readings and let us know what they are. Ammonia and nitrites should only be 0, nothing else is 'fine'.
A 40 gallon tank is a nice size to work with -- you have loads of options. Ideally, you would have to choose between tropical (gouramis) and cool water (goldies). Bettas and gouramies don't typically get along but it's easy enough to separate out the betta if push comes to shove. What sort of algae eaters do you have? Plecos? Chinese algae eaters? Otos? If they're plecos, very few plec species available in the average pet shop would do well with 2 in a 40 gallon -- common plecs are a disaster in that size tank and a bad idea with goldfish. CAEs are simply nasty fish with no real algae eating value. Goldfish typically need approximately 10 gallons apiece, so your tank would support 4 goldies, 5 with stringent maintenance if it's a "breeder" tank.
