With my betta I've always had luck with scissortail rasboras. A school of 4 or 5 is fine. The only thing is they get a little on the large side for a 20gal so I would recommend slightly more frequent water changes. But other than that they have striking markings but aren't too colorful as to aggravate my beta.
My Tiger Danios are starting to spread out into the middle. Tell me more about White Minnows - low maintenance? What is their diet like in your experience? While they run the tank, do they spend a lot of time in the middle?