Here you go but first a disclaimer. QT practices should be customized based on the species of fish. The simplest example is a Domino Damsel could handle more instability in water quality than say an Achilles tang. There are many factors that are species specific so keep that in mind. Having said that, I am going to answer your questions generally (at least as general as I'm comfortable with).
1 Wouldn't water change be very stressful for the fish, like 1 change per day?
Depends on how much of a water change, the total system volume, and the type of fish. Generally speaking, if you can afford to do water changes everyday I would think you can easily do up to 50% water changes on a daily basis and you probably wont have an issue at all. The smaller the system the more risk you may have but most likely there still will be no issue. If you change 50% of the water volume in a 10 gallon tank a more sensitive fish may respond poorly BUT only if you dumped all 5 gallons on top of them at once. If you added the new water at a reasonable rate and made sure the temp was comparable (PH too) I don't think you'd have a problem at all. Daily water changes is overkill IMO.
2 How much of the tank water should change every day?
Again this would depend on the type of fish, quantity of fish, and system volume. I don't think there is an accurate general response to this question. If you had a 10 gallon tank with one domino damsel and insisted on doing a daily water change I would probably do about a gallon a day or maybe even a half gallon a day. Again, daily water changes is overkill IMO.
3 Will a skimmer help to maintain good water quality, against ammonia, nitrit and nitrat?
Yes this would help in a hypo system but if you were using meds the skimmer is going to skim them out and you would lose the benefits of the medication (not too mention waste your money). On a hypo system, skimming is perfect. I run hypo with live rock, Sand, carbon, and skimmer.
