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If the Molly didn't get him the Black Skirt likely would have nipped his fins to shreds anyways. The Kuhlis would probably be a good Betta tank mate but the others, as you found out, not so much.
Always do your research and if you're told something is unlikely to work, as in this scenario, there is no shame in trying* on the assumption you have a backup plan.
Whiteskirts aren't as nippy as the blackskirts, many many people that I know that keep bettas in a community recommend whiteskirts. I myself prefer a little more color and keep my betta with ember tetras.