I'm not sure what is the best course of action. sometimes QT can be more stressful for new fish, and you know you already have some disease.
It's good to know the fish are small, so the bioload isn't too high.
I'm no disease expert, though I have some experience successfully treating disease. I've also had failures. I had a complete tank wipeout in 2005 because I did not QT fish I assumed were healthy. kind of like your neighbor's tang, I took established fish from my mom's tank and added to my established tank. didn't notice until it was too late that they had velvet (oodinium).
you might want to get the new fish slowly acclimated to fresh salt water the same temp, sg and pH of your tank water, in a bucket, then treat in a formalin bath for the recommended time before adding to your display tank.
also, be prepared to do frequent, large partial water changes in your dt. it may be a lot of work, but it will be worth it to keep your fish healthy.
Here's a good two-part article that I have found very helpful:
http://www.reefkeeping.com/issues/2003-08/sp/index.php
http://reefkeeping.com/issues/2003-10/sp/feature/index.php
I wish I felt more sure of what is the best course of action. let's see what others might suggest.
It's not ich, but I had another outbreak of oodinium about a year ago. I treated the display with chloroquine phosphate (effective for oodinium, not sure about ich - see the oodinium article by S. Pro on Reefkeeping as well). I did DAILY netting and hourly dips in formalin/malachite green for the two sickest fish, my dogface puffer and pink tail trigger. I did the dips for at least two weeks. I also fed medicated foods. the fish were looking awful, but both made it and are doing fine now.