keep in mind that bettas do not need aerators, but the tank mates you get will. You need a filter if you have tank mates. You do not if you have just the betta (most opt for it to break surface tension; keeps scum down; reduces the rate of required water changes.
If you have a tank that is wider than tall, you could potentially have a group of 4 Neon/Glowlight Tetra, or similarly small schooling fish. But that will be it. They will want the plants to hide in, and your betta will be less stressed to not see these tankmaes clear as day (that is what stresses fish out, is how many fish they can clearly see that alarm them).