I wouldn't worry about the goldfish spreading diseases if the fish was healthy in the first place.
But I would worry about the health of your goldfish being compromised by the higher temps required of tropicals. Or vise versa. Tropical fish do optimally at temps of 76oF to 78oF, and goldfish love temps at the 68oF to 70oF range. It seems perhaps only a small differerance to us, but it makes a major differance to the fish.
Goldfish suffer lethargy and suffocation at higher temps simply because they cannot get enough oxygen. Being coldwater fish, they require lots of O2 for their metabolic processes. This evolved from the fact that cold water holds more O2 in solution than warm water. Tropical fish, being warm water fish, do not have as great an O2 demand because they evolved in less O2 saturated water. Cold water also slows down the metabolism of tropical fish, sometimes putting them into a pseudhibernative state they can starve from.
You will hear of people who have accomplished keeping goldfish with tropicals, and I will admit, it does work, sometimes, and I currently have three fancy goldfish with a handful of danios(long finned blue) and tetras(bloodfin and blk skirts) in a 20gallon tank. The tetras have been as low as 68oF and the goldfish have done just fine at 72oF the range of my tank (it is unheated). In fact, everybody was doing fine until the danios were introduced. Unbeknownst to me (and my own stupidity), I didn't quarinteen the danios, and they gave my black moor fin rot (as well as all coming down with it themselves). I knew it wasn't the other way around because I've had the moor for a year and a half now with no disease outbreak.
Is it reccommended that you keep goldfish and tropicals mixed? No. Definately not in highly populated warm water tanks. But, if you do wish to keep your fancy goldfish, I would get rid of the tetras, mollies, and guppies, and probably purchase a school of six to eight white clouds instead. White clouds are truely a cold water fish and do great in room temp tanks. Danios also seem to do very well in cooler tanks, and can add some lively action to the slow ploddings of a goldfish. But danios can also tend to be nippy, and I'm wondering if that is what's causing the sudden infection of fin rot in my tank.
~~Colesea