The only real plus with the neons would be that they are faster. As Caleb mentioned their colors could be a problem, but I could see a molly vs betta death fight ending a lot messier than a few neons. If you do neons I wouldn't do a solo one, I would try 4 to 5 to get the group effect going and possibly he will leave them alone.
For reference I had one betta who lived happily in a 20 tall with sword tails and other peaceful community fish, then I had a dragonscale who literally wouldn't even tolerate snails. He would pick at their head any time they moved. Just keep your receipt and be ready to adjust accordingly. If you keep up regular water changes you could try the molly as well (not at the same time).
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