While both types of fish are relatively easy-going and often times either schooling or shoaling, they are way too different to be placed in the same tank with each other.
For one, goldfish get very large and increasingly messy as they grow. On the other hand, most tropical community fish stay small in comparison, and they are sensitive to large amounts of ammonia in their water. This alone should be a dead give-away that goldies and trops are probably going to give you a headache!
Next, each type requires a different temperature of water to thrive in the aquaria. If you aren't keeping your fish within their optimal temperature range they won't grow as well, show their best colors or breed for you. Quite simply, they won't be as healthy as the could be. Sure, they may survive for quite some time in a mixed tank. However, neither fish will thrive together.
Your goldfish is constantly looking for food! This becomes a problem once your goldie outgrows your tropical fish, like mollies and guppies. Small, tropical fish start looking like very appetizing snacks around this time! (Yes, your goldfish CAN and WILL eat other fish if given the opportunity. This DOES NOT mean that it is healthy for them, so don't start getting all excited to watch your goldfish eat live feeders.)
Speaking of food, their respective diets are WAY different. If you want to avoid a lot of the common fish problems associated with goldfish you should first start by offering them the proper diet.
So, this means:
1. NO mollies
2. NO platies
3. NO guppies
4. NO gouramis
5. NO tropical cat fish, like cory cats
You're getting the picture now, right?