Keep in mind that some bettas do not do well with schooling fish as it stress them out. I would definitely suggest trying it out, but just be aware of this.
I like zebra danios (they are super active swimmers and a 20g long is considered the minimum tank size for a group of 6)
The only tetra I would suggest are ember tetras. They are kind of uncommon, but are very tiny and cute. (you could put 15 of them in your tank easily)
The schooling species that I keep in my 20g long is the harlequin rasbora. They are very hardy yet fun and colorful. (I keep 12 of them, but you could do a few more than that)
You could do glofish (the danio species), but I am always very wary of anything that is "genetically modified". Plus, I think they look tacky haha.
As for a bottom feeder I don't know much about the dwarf plecos, but I do love cories. My favorite is the panda species, but they are a bit harder to find.
20g long NPT: 3 german blue rams, 1 powder blue dwarf gourami, 12 harlequin rasboras, and 2 otos
20g regular PT: 9 female bettas, 6 dwarf cories, and 2 mystery snails
10g divided PT: 1 male DT Halfmoon betta and 1 male Super Delta elephant ear
2.5g: 1 male Halfmoon betta