How long has the betta been with the other fish? My Leo was the sweetest thing ever, then after a month randomly started taking out all the corydoras. So now he lives on his own!
Don't get just one danio, get at least five if you want them to be happy. I think you'd also need a bigger tank as they are extremely active and love to zip around. If you get the bigger tank then yeah, get the group of danios. They might shred your poor betta's fins though, just keep an eye on him and have a small tank ready incase something does go wrong.