I have a male and a female Betta in a heavily planted 29G tank along with about 12 ghost shrimp. They completely ignore the ghost shrimp. The male occasionally flares up at the female, but they both completely ignore the shrimp. I had 3 guppies in there with them, that lasted a few hours before my Betta turned into Pac-Man and chomped em all down.
In the wild, 2 males will always fight over territory. It's hard-wired into their brains, they can't help it. But in the wild it rarely results in death, because the loser will always leave. Since that can't happen in a tank, Betta fights in captivity almost always result in death.