I agree. I once had a 45 gallon with three silverdollars. They never bothered any of the other fish. They grew to 4 inches. And were beautiful!Silverdollars (spotted, regular grow to 5'') redhooks grow to 12'', sevrums, or uarus.
I'm pretty sure nyc was joking.
And to the OP, for your 55, I agree with all the others who have suggested severums.