This seems to me to be much the same as your other thread on the 18g tank...if it is, I can merge the two. It is better to keep one thread for a topic, otherwise it can be more difficult for members to remain on track, and seeing the other responses often helps more.
Having said that, on to your question. I would not put Black Ruby Barb in a 20g; if you note in the profile it says this fish reaches 2.5 inches and should be kept in a group of at least 6, which means a 30-inch tank minimum to accommodate its more active swimming. You can see the profile of a fish by clicking on the shaded name in posts, or use the "Tropical Fish Profiles" tab in the blue bar.
The Celestial Pearl Danio is a miniature fish at under 3/4 of an inch; not a good match for a 2.5 inch barb.
A Betta is not advisable with other fish in so small a space; I assume you would want a male for the beautiful fins; they can be very aggressive--not always, true, but it is in the nature of the fish and you can't change fish nature any more than human nature
. I would not include a Betta in a community 20g.
Corys are fine, and 5 is a good number. Tetra is fine, depending upon the fish species selected; some remain smallish and in a group of 6+ would be fine; others need more swimming space and would not be suitable. Similar to Tetra, are the Rasbora in the Trigonostigma genus, like the Harlequin Rasbora, Lambchop Rasbora and Hengels Rasbora. A group of 5-6 of one of these would be nice. Check their profiles.