What kind of fish can I add to this tank?

We get this question a lot here at petfish.net. While a very valid question, it's hard to answer. There are at least several hundred possible combinations of fish and plants for any given tank. I strongly advise that you browse some pictures online, or better yet, look around your local fish store for things that appeal to you. Then, you can come back  to our articles (http://www.petfish.net/kb/1/) or do a Google search on the common and species name. Look at the general requirements, mainly tank size. Note that almost all tetras and most danios are schoolers, and should really be kept in groups of 6 or more. Just about any fish listed as "peaceful" or "community" will get along with others in the tank. Wardley fish (http://www.wardley-fish.com/friends.asp) has built a good little program to help you find which fish will get along with which. This site has a very good collection of fish descriptions as well.
To find out what kinds of fish are suited to your tank size, I highly recommend Boeing Dude's Minimum Tank Size List (http://www.members.tripod.com/~boeing_dude/id118.htm).
Betta compatibility is the most common question. This CANNOT be answered. Each and every Betta is different. You just have to try. Of course, you should avoid any kind of fin nipper, but otherwise, it's just chance.