the petstores say that so people will buy more fish there are many other factors then just how many gallons are suitable you need to understand bettas are territorial and may not like a friend in such a small space or vice versa. But if say you got a 5 gallon one other fish might be ok. but I would wait until you have a bigger tank. Or you could find a new female as a neighbour again even
A lot of tropical species are also more delicate than bettas though bettas should have a heater and all that but can be hardy sometimes without one...a lot of other species it is a must or they will not survive at all. So you have to think if your set up is good for other tropical species as well.
I hope this helps