no, because if you had proper schools of all the fish then there would be no space, also. mollies and bettas are generally a bad mix due to the mollies color, and size
I would have immediately said yes. I have kept betta splendens with no trouble at all with mollies, cories and tetras, but I must agree with FF2. If you have so many fish in your tank to fill out the minimum numbers for schooling, it may not leave enough room for the betta. I would at least establish a stable tank with your other fish before trying to add the betta.