I heard something about danios needing a predator or something for them to school together. Or else they don't.
For them to school tight they will have to really feel threatened. They wont care about a betta. you want something Like these guys
Minnows have never schooled tighter than when thrown in this tank.
I tend to like the more stream lined fish so I'd go with tetras. I like neons, bloodfins, Colombian red blues, pristella with tetras possibilities seem endless.