] Yes you are ignoring are advice.
Chocolate, please be careful how you say things to others.. we are here to help, answer questions, give advice.. but we are not here to enforce others to use our advice, nor make them out to be a bad pet owner if they choose not to.
Making people feel bad for how they care for their animals will only drive them away, and if they aren't here, they won't learn what they came here to learn in the first place. You can't force people to listen.. and you shouldn't badger them about what they should be doing. You have your opinions and ideas, while others have theirs.
Sorry, was walking out the door when I saw this thread and posted in haste..
I agree with Phaydra, observe and see what happens.
I know there are guidelines and preferences when it comes to most fish (some do need certain environments like the discus, etc), but generally I have always followed my instincts and any research I have come across I take with a grain of salt - I only follow certain things such as temperment (to a point), and temperature, etc. Over the last 20 years or so I have kept questionable fish together peacefully and kept them alive healthy for years. It's finding out what makes them comfortable.. do they like a lot of plants? Caves? etc. It's keeping them well fed with quality food to make them not need to search out and make prey of other fish. It's water quality. And it's the temperment of the individual fish itself that plays a big role in things too.
In a 30 gallon when I was 17 or so I had a big black lace angel fish.. hated being with other angels, but would not bother any other fish, including the small schooling fish such as neons. He ate live ghost shrimps that I kept in the tank, along with other frozen/life foods a few times a day. Very interactive with me and would even dance in a certain corner whenever I came into the room, such as a betta. He was very happy with docile fish, and semi aggressive fish. They had good food, clean warm water and plenty of hiding places. But many angel enthusiasts would shake their heads at me for keeping him in his own tank away from my other angels, and with smaller fish like neons or tetras, etc. But he never killed or harmed another fish, the other fish left him alone - never a nipped fin. A mix of his personality and a good home.
Personally I would follow your instincts as well when it comes to your dalmation molly - yes, they prefer brackish, but I never had kept them in brackish and mine all had lived years just fine and healthy (not trying to sound better than anyone, or that I am perfect.. but being totally honest, in the last 20 years I have had only one mass sickness which was ich when I first started and had to learn through books at the library. I had only a handful of deaths (less than a dozen) all together - most of my fish from angels to oscars had been sold to stores or rehomed with others due to moving to a different state or space issues, etc). I may be relatively new to bettas, but in the last 18+ months I have drowned myself in how to care for them properly and how to home them. And now I feel comfortable at trusting my knowledge when giving advice.
Male live bearers tend to be a bit more aggressive.. or it seems to look that way, but generally they just want to get.. well.. laid and focus on that more than anything else. Females are a bit more docile.. unsure whether yours is a male/female, didn't see it if you said. The only problem that could arise will depend mainly on whether your molly is of the sailfin variety.. if so then it could be an issue as a male betta may see the sailfin as a threat or show of dominance if the molly extends it fully.. if he is just the average dalmation molly, then they will most likely not pay each other mind for the most part.. male bettas will tend to be more territorial at the top of the tank when it comes to a community tank, and mollies are all over but generally middle.
So in all honesty.. keep your molly, your betta, and 3-5 cories on the bottom are fine - that is a good set up. Keep a decent amount of plants in there (silk or real), a cave or two as the betta may enjoy that.. and see how it goes. I would say things will be fine as long as your betta tolerates other fish.