Danios are really easy to breed, giant danios would probably look nice in a 55g.
Rainbows would look much nicer, and I know they can be bred in captivity. If you go with rainbows for a 55g, you'd want the larger ones since you'd see them better. Also, they can interbreed if they are similar types, so if you are going to breed them to resell try to stick with all one type of rainbow. Of course, a school of larger sized rainbows is kinda expensive...