Smaller is all relative. Even if a female Cr. sp. "Xingu I" is smaller than a male it still can exceed 14" in length! That's a big fish for a 55 gallon tank. Hard for the fish to turn around when the tank is only 12" wide!!
With that said, I think you can do this if you have excellent filtration and if you perform large scale regular water changes. At least 50% once a week. I'd still feel better if you were attempting this in a 75 gallon tank that is a few inches wider.
Certainly you shouldn't plan on having any tankmates...