I would not consider this tank over stocked in any way bio load wise. With the possibility it is more so a tank with fish that aren't best suited for that environment, as stating this tank to as being over stocked would be like saying keeping a Cory in a group of 1 is overstocked. Man Of fish. I don't know your definition for over stocking but mine would be more so along the lines of having to many surface, bottom, mid dwelling fish, having too many fish, having to many territorial fish, etc. not ones ability to swim a long ways.
Now to the original question
With your tank you are already encountering problems referring to the stocking. As danios, as well with those types of minnows. They are both schooling fish, which means they can live in smaller groups as small where you have them yet they won't be as secure and you won't see there full potential keeping them confined to lower numbers. You also have another problem with the danios, they are very active fish for there size. And won't be as happy in anything short of a 3 foot tank due to there activity and group preference.
If you could ideally find a better home for the danios you could easily purchase a male Betta, assuming you have no flashy fish, and no plastic plants. And you are willing to get another minnow
Also
From expecting experiences a Betta can survive in unaceptable conditions such as keeping them in confined spaces with minimal water changes, but mark my word. They Survive, ideally with all fish you want them to thrive which is why a tank upwards of 2.5 gallons would be ideal, more so of a tank around 5 gallons would be much better, and you'll see the full potential in the fish regarding to behaviour, and personality