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. Baby Plecos would also render the tank over stcoked. Any type of Pleco would outgrow a 10 gallon easily, referring to the more common types such as bristlenoses, and pit bulls. My advise would also to be return the Pleco as well
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
and the wcmm aren't really suitable.
