Personally, I would leave him on his own. I have a betta in a 30 litre tank with pygmy corys. They get along okay, but after a few months I did notice my betta become more territorial and he does sometimes chase them. It's a small tank so he thinks it all belongs to him. It's not serious enough for me to move him, but it's not like he benefits from their company. Also, he does like to eat their food. Greedy fish.