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To be honest I'm stuck at the betta as best choice.
You may see some small fish at the lfs of an inch or so. The smallest here available is probably neon tetras but by the time you make a small school, factor in small tanks and small fish can be difficult - back at the betta again. Especially in a tall tank.
One other thought would shrimp. They are sold at more specialised shops in our area but we had a guy present on these one club night and there are some amazing colours out there.
Chilli rasboras sound nice. I don't think I've seen them here.
I have to agree with Delapool. Stick with the betta. I haven't had much luck with shrimp, but my water is very hard with a high PH. You could do a small school (3 or 4 max) of smaller sized cories to add action to the tank. No single cories though; they prefer the company of their own kind.