WIth a betta, you are looking for a fish without colorful finage to attract the betta's biting. Also you don't want fish that are nippy to the betta. There are a lot of threads on this, if you use the search feature it will pull up lots of results. And my sig has some and I have kept mollies with them as well. Just take caution, a betta could act up, and you need to keep an eye on things in the beginning.
I have heard 6 is needed for schooling fish. I have always had 4 cories, and then the loaches kind of join into the school. They all seem healthy and happy.
I don't think sand is a necessity, if that is something you're not into. Although I have read that it is better for loaches' barbles- and I'd suppose other's. Mine seem no worse for the wear with gravel. It is smooth medium sized gravel.
If you are concerned about yo
ur bioload, you can go with pygmy cories.