Without trying to stir up any trouble in the fish section, may I ask a genuine question about tank mates?

First, let me state that I have not been without an aquarium or several aquariums for about 45 years and also have a VERY large Koi and Goldfish pond. OK, here's the question. I have a cold water tank in my family room. Several months ago, my neighbors girlfriend was desperate to get rid of a few of her small tropical fish, so they were dumped in my cold water tank. There is one Cory cat and several different small tetras. While I KNOW that different fish have different requirements, and there is NO heater in my tank, WHY have all of these fish been doing so well for months, without any problems whatsoever? As you can see, I'm a TC in dogs, not fish, but every time I even come to the fish section to answer a question, I try to give good advice, including that Goldfish and Tropicals can not be tank mates. So, is it the luck of the draw, or are some of these fish just hardier than we think they are? Just to let you know, my furnace is set on 69 degrees F. if that happens to make a difference, and my tank is a 55 gallon with an AquaClear 110 filter. All the fish get fed Tetra Tropical fish crisps right now.