I really think you should swap the 90 gallon and put the venustus in it. A big fish needs a big tank to move in. If you have not bought the tank yet, the longer the tank the better, length is more important than height or even gallons really. In a larger tank he might even tolerate a couple females, thats the key for africans, more than one girlfriend LOL. Go for maybe a trio or quad. (one male and 2 or 3 females). Having more than one spreads his aggression so one fish wont get all of it. Include lots of rock caves for hiding spaces.
If you feel guilty about your pet goldfish, why dont you just redo their 55 gallon tank. Buy new gravel, plants, decorations, even a new filter for them. Make their tank brand new. Make it really special. 7 goldfish are just fine in a 55 gallon, no problem there at all. A gold fish is not likely to reach 10-12 inches like a venustus will. It is much more than they need and much more than any goldfish i have ever heard of. Your venustus might really need the extra space of your 90. Just my two cents.
If you feel guilty about your pet goldfish, why dont you just redo their 55 gallon tank. Buy new gravel, plants, decorations, even a new filter for them. Make their tank brand new. Make it really special. 7 goldfish are just fine in a 55 gallon, no problem there at all. A gold fish is not likely to reach 10-12 inches like a venustus will. It is much more than they need and much more than any goldfish i have ever heard of. Your venustus might really need the extra space of your 90. Just my two cents.

in the first moment i see a 10'' male venustus. he start to talk me about cichlids and other incredible fish..
but still love em