I would reccommend a larger tank for giant danios, and more of them. They like to swim in groups. My zebra danios race back and forth in my 35 gallon and they stay a fair bit smaller than giant danios (I have six of them).
For a 20 gallon, go for something like 4 corydoras catfish, some dwarf gouramis, or a larger gourami, and some small tetras like neons or glowlights (aim for about 7 of them). That would be a nice, peaceful community, something like one I used to have.
That tank had 3 corydoras trillineatus, 5 cardinal tetras, some guppies, and a blue gourami. That gourami gets a bit big though, maybe try and get some dwarf types.
I had a pair spawn in my 35 gallon about 2 years ago, which really surpirsed me. I had to go away for the weekend, and when I came back, there was no sign of any fry.
Anyway, good luck with your tank, hope all goes well.