Feeding is prob the biggest problem when mixing tropicals & goldies. The temp is not that much different as fancy goldfish are actually sub-tropical, so will overlap quite a bit with many tropical fish.
Since goldies & tropicals have different diet requirements, it is difficult to make sure each get the correct food in a mix tank. <The goldies will eat everything, sometimes getting into gut problems when they eat too many tropical flakes.>
One way is to have separate feeding area. It is possible to train the goldfish to go to one spot for food. <Just feed them at the same spot every time.> If you use say sinking food for the goldies, then while the goldies are busy at the bottom feeding, feed the other fish their flakes at the opposite corner of the tank ... they might get some before the goldies finish ...
Now if you have commons & small tropicals, they will have the tropicals for lunch when they get bigger.