There is a chance your molly might be nippy and want to go after your betta's tail. If your betta is of the really expensive type, like a nice half-moon, I'd say no go. If it was only a pet-store vailtail I would say try it.
Sometimes it works, sometimes it doesn't, so there is no hard-fast answer to your question.
The only truth to the matter is like Vyache said, mollies and platies like to have a little salt to their tank while bettas do not. Given this different preferance in water chemistry, I would personally not keep them together. Consider purchasing another dalmation molly (of the opposite sex of your current one) and another red wagtail platy (of the opposite sex of your current one). Mollies and platties do their best in trios of two females for every one male, but you can occassionally do well with just pairs. If both your molly and platy are female, then definatly keep only females. This will prevent breeding and help reduce mating aggression. But try not to keep a tank of males, they will often fight, a lot.
~~Colesea