you ought to pass on the powder blues; your tank isn't big enough for the cichlids you have already! is the 55g a permanent home or were you intending to upgrade at some point?
A 55 gallon is enough for an Oscar or a Texas. That's about it. Eventually the Texas' will kill the Oscar, I have no doubts about it. Once they do that, unless you get really lucky and they become a mated pair, they'll turn on each other.