Hey!
So, it is really easy to tell when a betta is stressed.
Stocking:
Mollies and Bettas both have long fins. No one has said why they wouldn't go well together. Unless you start seeing some aggression, especially flaring from your betta, you needn't worry about it. Never move a fish around more than it has to be.
Water Parameters:
Looking at the pH differences for these two fish, I think that'll be your downfall. Mollies need high pH, upwards of 7.4-7.6. Their temps are similar to bettas. Bettas are tropical fish and mollies are freshwater/brackish fish.
Betta pH needs to be low, around 6.4-7.0. Too low for mollies.
Space:
Depending on what mollies you got, some people will say that 10g is too small for mollies. Ordinary mollies grow around 3 inches long while some can grow to 5 inches long. In that case, you would need a bigger tank. If you have smaller mollies or male mollies, your 10g is just fine (I have four mollies and three platys in a 11.5g and have had 0 problems). If you can keep your tank clean, nitrite and ammonia levels down, then there shouldn't be a problem.
My questions for you:
What breed of mollies do you have? This helps us dictate space requirements.
Have you noticed any aggression from mollies or bettas?
Are your betta's fins ripped, same with your mollies?
Water parameters?