PH Question

I have always floated newly purchased fish in my tanks to equalize the temperature and then mixed some aquarium water to equalize the PH. My question is for a more defined approach. How much of a PH change and in what length of time is it safe for the majority of fish? If the Ph is 5.8 for the new arrivals and the tank has a Ph of 7.4, how long does it take to make the change. I have been told that it is unsafe to move a fish when the Ph difference is 0.5 or more. Is .25 safe or stressful? Can I make a 0.25 change every hour, every two hours, or every 24 hours? Does anyone have a definitive answer that would be safe for the majority of fish breeds?