Some of the bigger ones need bigger tanks. Mollies are not the calmest fish. Mollies do best in brackish water. Platies are too big. Bettas might not like Neons.
Ive seen SEVERAL bettas live calmly amongst neons. And as i said before. Bigger mollies require atleast 20 gals. And no they do NOT need brackish waters. They do well in either waters. And platies dont get any bigger than mollies. I stand by what i said. If you think otherwise. PM me links that back up your opinions.
I too like cories, but keep in mind that everyone has their own way of setting up a tank and even if they have the space, some people might not want other fish species or invertebrates in the tank with their betta. :)
The guy who helped me pick out the tank at Petco said I could find a tankmate for him, but I honestly believe I am going to just let him have the 10 gal all to himself. I only have a couple years of experience and I don't want to take the risk of stressing him out or him and his tank mate fighting and one of them getting hurt.