Some of my platies gets along well with my betta's
Some don't, They like to follow behind and grab on the long fins, like its food or something.
Now with female bettas they all get alone fine, it's just the males,
Two days ago I watched one of my bigger bettas chow down on one of my tetras. I have two bettas in a divided tank but the tetras can get through to both sides. By the end of the day there was nothing left of that poor little tetra. I believe there is another thread about combining bettas with tetras.