I don't know anything about "tetra disease" but I know that the recommended time to QT anything is 4 weeks, and by then just about everythig they might have will show up. But most people do 2 weeks just because they can't wait that long. If you can I'd try 4. But 4 weeks with 6 fish in a 2.5 is a lot of water changes.
For the heating, I use a 100w in my 26 and it works great, so I'd suggest 100-150.
Cory's are cute aren't they? :)