You can float your fish for a while. If its been bagged for a long time, open the top and try to get it somewhere where it won't tip over (or clip it to the side of your tank. Adding water during acclimation should provide oxygen.
If you hypo the QT tank, odds are it will wipe out whatever Ich is present on the first Tang and hopefully the new fish won't get it. If they do, hypo will get that too.
I would slow it down a little. adding three fish at a time could be overwhelming to your QT (or even a smaller DT). I usually won't go more than a fish or two a month. Gives time to watch them in QT and treat if something shows up. Ordering fish before the original fish is clear brings up this kind of scenario.
+1 to getting PVC
for fish to hide in. Places like Home Depot sell it.
I disagree that all tanks have Ich. They only have it if you allow them too. If you set up a tank, it can't magically have a parasite or disease. If you properly QT fish prior to adding them to your DT, the odds are fairly low that one will sneak through with something not being observed first. Some people run their QT hypo just in case. that knocks out some of the nasties out there. Systems with Ich can often do well, if there is proper diet, low stress and everything is kept in check.