All live rock does the same thing, it's just named by where it comes from and sometimes the shape of it too. It's up to what you think looks the best. Keep in mind that if you get live rock shipped (which is much cheaper, seeing as you'll need 45-60lbs for the tank), it'll have some die off during shipping and you'll have to cure it again a little when you get it home. This isn't too much of a problem though, since you aren't going to add it to an established tank so it can just finish curing right in the 55g.
Damsels are very, very territorial (and I think that dominos change colors when they mature and become dull solid brown). The blue tang gets too big for a 55g, but a yellow is fine. You will want to wait until the tank is completely cycled and the clean-up crew has been established for a while before adding fish, so you've got plenty of time to decide on final stocking.