From first hand experience keeping a goldfish at tropical temps (72-76 etc), it can work but it's not ideal. The goldfish will have a shorter life and will be more prone to getting sick easier because of the temp difference it needs.
The reason I say it will work is I was able to have my black moor (bought from walmart of all places) for 6 years, 8 moves, and 4 states at 70+ temps. I didnt know better at the time and since Ive always kept him at those temps and he was doing so great I thought nothing of it but I basically cut his life in half.
Temp might not seem like a big deal but we have to remember that their bodies will react differently in different temps, leading to things like faster metabolism, heartbeats, etc. Just like if we were outside in 100F days all the time, sure we can do it but it puts a lot of stress on our bodies that arent always apparent.
I would move him into a tank that is setup solely for ideal goldfish conditions when you get a chance.
40Gal Planted- 10Gal RCS Planted
6 Julii corys, 1m Furcata Rainbows, 2f 1m Bose Rainbow, 2 mystery snail