I think you did everything right. As long as the newt is eating and does not show any other signs of infection (swelling, anorexia, etc.) Quarantining your pets is alway prudent, unless you will add additional stress on the newt. In your case, if his current environement is optimal, then he should be okay. Some keepers get quarantine backward when one newt gets infected...they remove the sick animal, leaving the others in the same environment.
If you really want to deter communicable infections/disease, you would remove the others as well. This is for general fungus/infections. The old set up needs to be broken down and started over, disposing of anything that can not be disinfected safely. Of course this is for extreme situations. I had a setup that was infected with nasty bacteria passed on by live food source. After failure to salvage all the newts after quarintining, I had to ditch most of everything except the tank. Talking to tropical fish enthusiest also recommend discarding gravel completely (especially colored gravel). Boiling can destroy sealant on colored gravel, I was told.
That is one bonus on having simple setups with no gravel
