It is never a 100% sure thing with a betta. He should be fine with cory cats or other bottom feeders. Mollies are top dwelling fish and will be in his territory a lot, this could be an issue, also, mollies can be very fin nippy, wich is a huge problem with male bettas. Guppies can be fin nippy and fancy guppies could be mistaken for rival bettas, They are not a good tankmate for bettas.
I kind of nervous because now after i did that post i am afraid that many people will have problem. How i understand it always a risk with bettas. They can be good at the beginning but can get aggressive later. So please be careful guys. May be before you do any changes to your tank take a few opinions. And i like opinions from the people who had the same situation not just research on interned.