A lot of smaller fish will be MUCH easier to take care of than one large fish. Predatory fish are messy eaters and water quality declines quickly. H2O changes are always important, but ever more so with large fish that are confined to a small tank.
Agree with the other posts about RBP in a 55gal. It's just to small, a 125 is decent. They can be very skittish and can knock the top off the tank. A friend had a trio in a 125 that grew to 8 inches (and thick too!) right at a years time.
To add to Damon's post about the 55 width, if you can go up to a 75 gal for the extra depth DO IT. To me a 55gal is kinda deceptive. It looks big, but you really can't do as much as you think you can with it. A 75 is much more versatile.
I think that I've read that you have a 55 already, just adding my thoughts to the subject.
