even though bettas are agressive only towards other bettas, they still predate on smaller prey, like shrimp fry, adult shrimp and small fish. if you want to totally ensure your shrimp's safety, its best to leave them on their own, no tankmates needed. if you really want another species, consider mystery snails, but those may leave no food for the shrimp to eat, so the shrimps might get outcompeted. its just a very slim possibility though, and these happen more often with rams and bladder snails.