Ich lives in the sandbed. When the fish sleep, which is often close to the sandbed, new ich tomites attach. By putting the fish in isolation, you are allowing the trophonts attached to the fish to drop off, but since there is no infested sandbed, new ones waiting to attach will be fewer in number or non-existent. In your Kole tang's case, reducing the number of attached parasites is your #1 priority, and that means letting the ones that are on there fall off in an environment where new ones don't take their place.
Ergo, getting a fish that is already fighting the infection into isolation ASAP is critical. The other fish can wait. Therefore, wait until your QT has cycled to add the other fish and start hypo. Even if the waiting period leaves the tang with no sign of infection, starting the hypo regardless is critical.
As for your levels, any readable ammonia level is dangerous.
That's why you need to do a large water change on the QT to drop that level down, and continue doing large water changes at least every other day to keep it from building until your filter can take care of it.
You can also use a product called Prime from Seachem to neutralize ammonia on the "off days" (the days that you aren't changing water). You just have to stay on top of it, that's all.
