a betta might actually work better in that situation. although some bettas can be aggressive to other fish. that's why it's hard to say. you could just do the tetras and cories and have a pretty tank.
i'd also say get more tetras, and more pygmy cories. they're schooling fish and do better in groups.
i'd also say get more tetras, and more pygmy cories. they're schooling fish and do better in groups.