I wouldn't because the activity of top dwelling danio's can be stressful to the betta if it's not a big tank, and rainbows get a little big but could work. To give it a shot though, wiith the fish you have, don't try it in less than a 20 gallon tank. Even with calm fish, it won't work in less than a 10 - betta's don't kill other fish, but they are territorial and we need to allow for space for this.
Either way, the people who say betta's 'kill all other fish' just follow dumb myths blindly. If only they came back to read some other answers, they might actually learn something instead of repeating nonsense.
