I'm not a big fan of putting bettas in with mollies or platies--they have higher pH, salinity, hardness preferences than bettas do, and while bettas can somewhat adapt to these conditions, their fins usually suffer. Platies can also be aggressive toward bettas, and because of their bright colors, that can go the other way too, and the betta may attack the platies.
It's good advice to just keep the betta by themselves in a 5g tank, believe me, bettas are so beautiful and so full of personality, they don't need of other fish. They are show-stoppers all on their own. :)
Another note, if you are going to keep corydoras catfish, they should be allowed to be in schools of six or more. They are very social fish, and if they are kept in smaller groups they tend to be insecure, stressed, and do not live as long.