Hi there, megoto. If she wants to stock the same type (black moors are considered fancies)... I'd be comfy with 4 of them in a 55 gallon. For stocking, we base this on 15 gallons per fish.. and that comes to 60 gallons. If she's interested in other varieties, the slower (wobbly) fancies are also good tankmates and are fairly interchangeable for stocking purposes. Suggestions such as orandas, fantails, ryukins, and other telescopes (like black moors).
I don't actually discourage some mixing between fancies and pond types (comets, commons, shubunkins, wakins). Just be sure to calculate those at 20 gallons per fish if you include any of these types.
The main setup difference to keep in mind for your wife's goldfish tank is to double up on filtration compared to normal freshwater tropical tanks.