When I got my first Goldfish, I kept him with my Guppies, Platys, Neon Tetras, Cherry Barbs, Dojos and Pleco. I think I may have even still had my Zebra Danios at the time. I got him because 1) I wanted to see how long he'd live ($0.15 feeder xD I'm terrible, I know. For the record, we're going on a year and he's doing fantastic.

) and 2) I had a snail problem. He was an inch long when I got him and lived fine with them until he was about 4in at which point he started ingesting my Guppies and so he was removed. They'd both go after the food and the Guppy would loose. XD Platies don't require salt. People say it can help them but I tried it once and lost a ton of my fish without even putting the full recomended dose. I'll never put it in my tanks again. :/ My Goldfish tank is about 76 degrees which is the same as all my tropical tanks. I don't do anything different for my tropicals than I do for my Goldies. *shrugs* The only other problem besides the fact he ate my Guppies was that as he got bigger, the waste amounts did too. All my fish have lived healthy, long lives in this temperature so I don't feel the need to change it. Why try to fix something if it isn't broken?
As for Platys being fast fish, they really aren't. All mine that I've had before (75+) were all mellow and just picked at the food on the top. They never raced to be the first to the food or were greedy. It'd be ok to mix for a while but you'd eventually need a new tank to seperate them down the road when the Goldfish got bigger.