What source of ammonia did you use for your fishless cycling?
If you used more than 4~5ppm ammonia daily to finish your cycling, it is not required to add a few fish at each time after the cycling was already completed. I fully stocked my 40 gallon tank with 30 or so fish at once. No problem at all, because I know the amount of ammonia I added for fishless cycling was more than a fully stocked tank can produce.
Those Danios need to be in larger group. It is recommended to have minimal 5~6 for each species. As you have 3 separated species, you need total 9 more to make group of 5 for each of them.
However, since you already have 11 fish in a 20 gallon tank, you don't have too much room left. If you really adding 9 more fish, it makes total 20 fish, even they are small size fish, you will need to increase the weekly partial water change to more than 30% to keep nitrate down.
