First, fishfish123, welcome to TFF.
Now, to begin with your food question...frozen foods are usually accepted by finicky fish as TekFish said, but this is a real problem. Frozen foods are not anywhere as nourishing as the prepared foods (flake or pellet). It is advisable to get fish eating prepared foods as their staple, to ensure good nutrition, and then use frozen foods as treats. Which brings me to the bloodworms...this is a food for no more than once a week. It is not nutritious, and there is something in it that in excess is problematical--fat I think, even though the level on the package might not suggest this. Anyway, not a staple food. Frozen daphnia would be better but again this is not as nutritious as prepared foods. Betta are surface fish, so you want foods that will float for a time, or sink very, very slowly. Your cories obviously need proper sinking tablets/pellets/disks; they cannot be maintained in good health from floating foods that eventually sink (mine won't touch these anyway).
Which brings me to the fish. In a 20g, you do not have room for much more, with what is there, after you increase the cories. The otos I would leave at 2 here for reasons I won't get into. But cories must have at least five, preferably more, but the tank limits this so five or six in total.
Mollies need more space than a 20g tank can provide; they will (if healthy) attain 3 inches for males, and females five and even six inches. This means a much larger space. Also, they must have moderately hard or harder water; they will not last long in soft or acidic water. The Betta, gourami, otos and cories are exactly opposite.
Bettas are not community fish. While individual fish may get along with other fish, or seem to, this does not always last. The inherent trait of a male Betta is to be "alone." It is risking things to force fish into an environment for which nature/evolution--or man in this case--has not programmed them.
Byron.
