Neon Tetras can be nippy, I've heard. I kept them alone so they were fine amongst themselves.
Scarlet Badis males fight a lot, again not from my experience.
Cardinal Tetras seem to be really peaceful. I would put my vote in for these.
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In my experience any schooling fish can nip if not given a big enough school. I've kept 10 neons tetras in a community tank for about 5 months now and they are the most peaceful thing in the tank aside from the corydora.