Goldfish, especially fancy goldfish, do often get raised in warm tanks - but not so warm as to be considered tropical. The reason that this is done is partly precisely because, being cold-blooded, fish have a higher metabolic rate at higher temperatures since reactions involved in metabolism (i.e. during respiration - the process that ultimately converts food to energy) go faster and, as a consequence, they grow faster.
This means several things - first of all, it means they would live shorter lives (remember that 20 years is pretty normal for a healthy goldfish!).
They also produce far more waste - as major poop machines, goldfish are already problematic in this respect - but this impacts on any other fish living with them and tropicals, because they have evolved to live in warm conditions anyway, are not affected like goldfish and so are used to far lower levels of waste. Keeping goldfish with tropicals is therefore bad for the tropicals as well.
The final issue is the one alluded to by the above poster. Just because a species has evolved a particular defence mechanism, doesn't mean the impact of using that defence is necessarily positive for the individual (extreme example would be a bee dying after stinging an enemy, less extreme is the increased likelihood of death after a lizard detaches its tail when attacked to distract the predator). Yes, many species of fish that live in bodies of water that vary in volume have the mechanism described (they release hormones which slow growth) to keep them from growing rapidly under poor conditions. However, in the wild, this would not be a permanent situation. Their ability to do this also doesn't mean it's not harmful in the long term (shorter lifespan and stress which makes them more prone to disease). Not to mention that fancy goldfish are not wild - they have been domesticated for a long time and, over this period, raising them in large ponds was the most common practice. They are not built to deal with 10 gallon tanks.
Finally, I'd like to point out that the hormones would only remain in the water in the first place if it isn't changes. In a lake, that's not an issue. In a small tank that is also severely overstocked AND at a higher temperature (high temperatures are also an issue in terms of the amount of dissolved oxygen BTW which correlates inversely meaning that it is worse overstocking tropical tanks for yet another reason) you have the need for more regular water changes. If you keep up with these, the fish will not be stunted anyway and you'll have the other problem - 10" goldfish that can't turn around in their own home. It's a catch 22 - the only humane option is to re-home them.
edit: and to the above poster, have you actually looked at your pond's temp in the summer? I don't know how warm it gets where you live but water has a very high specific heat capacity and does not increase in temperature as much as you'd think - a heated tank is very different.
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