I wouldn't do it. . . It would have to be a pretty big tank, wouldn't it? My understanding is that a pictus needs about a 75 gallon tank for manuevering when it's mature. I know they are suckermouths, but some catfish just love the slime coat on goldfish, and will "attach," much to the detriment of the goldfish.
Goldfish are bottom feeders, and don't need a bottom feeder to clean up after them. The pictus is also a lot faster to the feed trough than fancy goldfish. . .
Just some thoughts.