I know what you mean--Petsmart is the only one around here that carries them. Good that you didn't get them--you don't need fin rot to start out with.
Most people do recommend more females than males, but you could try 3 M and 3 F and see what happens. Since the ones they sell are generally juveniles, you may not notice a problem until they get more mature. That's when fighting would occur, if it's going to. So you could be fine for months, then suddenly have a problem. It's kind of a cr*p shoot. Some fish are more docile than others. Just hope you get easygoing ones! Also, it depends on who pairs up with whom. You could have two males fighting over one female. A lot of different scenarios are possible so there's really no solid answer, unfortunately.