+1 on just about everything that's been written so far. Just a couple of points:
When it comes to tank size, bigger is better. While this is true for the fish, it's also true for the fishkeeper. Bigger tanks have more stable water chemistry and are easier to maintain, something that might be especially true for fish that produce a fair amount of waste, like goldfish.
When it comes to mixing single and split-tail varieties, it perhaps isn't the ideal, but it can be done successfully. While I wouldn't start out the same way again, through a set of circumstances, my goldfish tank contains such a mix and with a little watchfulness around feeding time, nobody has a problem getting enough to eat.
Finally, when it comes to temperatures, it's true that goldies are coldwater fish, but they are very tolerant of a wide range of temperatures so long as the temperature change is relatively gradual. In my opinion, an air-conditioned house and/or a chiller isn't necessary for goldfish. We're in the middle of a heatwave where I live and our house isn't air conditioned. As a result, my tank temp's been in the upper 70's to mid 80's for a couple of weeks now and my fish are doing just fine.
Good luck with the goldies, I happen to love them too.