You need to keep at least two of every kind of rainbow you get. They're much happier with a friend to play with! With the other things you have listed, and your tank, you could probably have pairs or trios of 3 or more kinds of rainbow, depending on which ones you choose. Some of the little ones like threadfins or Pseudomugil signifer, you could keep a large school. But definitely get more than one. The more there are, the more active they will be. You could probably also do more than 4 danios if you wanted. Otherwise it sounds like a great tank, and you should definitely get some pictures on here when you get it going.