Seems like you are in a bit of a flap first off if it is hex, and it sounds likely, the time is on our side.
If I've understood correctly the affected fish is in a tank on its own and being treated with metro and high temps?
The existing fish are still in the original tank and are being treated with metro in food? and high temps? If so I wouldn't have done this without symptoms, and if I was to do this I would have treated them all together. Now that you've started the course (if I've understood it right) then finish it, but keep the temps normal in the main tank.
If you haven't treated the new fish in qt then don't if they are not showing any signs. Keep seperate nets, pipes and towels for the qt tank and it should be ok.
For the obviously sick one yes metro at 400mg per 10 gallons twice a day, keep up with w/c's and keep the tank as clean as possible. Feed only something like fbw and sparringly, remove eneaten food after 30 mins. Its up to you about the temps, I've had the same level of success at normal temps (84 to 86f or 29 to 30C) as I have had at higher temps. The one thing higher temps wuill help with, or may hep with si the loss of appetite, it can stimulate the fish into feeding.
hth,