I would say it is just because the bettas are some-what aggressive with other fish( exspecially the males) and if it is one gallon per fish, i mollie is on average 1 to 2 inches, and bettas are usually 2 inches so that would add up about about 11 gallons if you said that the mollies were 1 and a half inches. And the bettas are semi-aggressive. So, I would say keep the mollies and get rid of the betta and your set!!!!!!!!!!!!!
Mollies, 1 to 2"? I don't know where your getting your info from but its wrong, I think balloons might average off at about 2", however normal sized mollies are usually 4", 5", even 6", depending on what type they are. Too big for a 10 gallon (as its already been pointed out).
hakisuma_chan, a 20 gallon long shouldn't be too much, if you go with a used tank you'll save LOADS of money. If you buy everything brand new your looking at over $100. So defently go the 2nd hand route, you could even find tanks for free quite easily! A 20 gallon long is 30" by 12" by 13". I love mine, the extra length is great and IMO they look larger then a standard 20. You also have more surface area so can have more fish then in a normal 20 gallon.