Hyposalinity is the best treatment for ich. Safe for fish, non toxic, etc. Cooper works as well, but it is a toxic metal to both fish as well as the parasite. It should only be used when no better treatment is available, which is not the case with ich.
Yes, you will need to hypo all fish as ich is highly contagious. Also, the ich will remain in the tank if you don't remove the parasites' "host", which is the fish, for at least a month. Thus, all fish must be hypo-ed.
The procedures mentioned for setting up a QT will give your QT a kick-start, however, the biofilter will be very weak. Feed your fish sparingly, vacuum up all uneaten foods. Any filter pads/media you use, place in your main tank so that it can get saturated with nitrifying bacteria. You will need to test your QT daily for ammonia and nitrite. The first 2 days in QT will likely not be a problem, as you will be doing mass water changes to drop the SG to hypo levels. However, after that, you will need to really pay attention to your tank's cycle. Do not use a swing arm hydrometer for hypo. They are so inaccurate that they probably shouldn’t even be used for any reason at all. If you do not have a refractormeter or a salinity monitor [best choices, and should be on your “next to get” list of equipment if you don’t already have one] then use a high quality glass hydrometer.
Water circulation and water quality in the hospital tank is just as important as the treatment itself.
Also, offer your fish garlic soaked food while they are being treated.
