Well, I got the corys transferred. The PH was closer than I thought and all the other water parameters were good. I did a 25% water change on the 75g, bringing the nitrates under 5.0ppm. everyone seems to be happy but I'll be keeping a closer eye on them for the next few days.
I decided to upgrade the filtration on the 20g quarantine tank. Ordered the smallest Eheim classic, and a 9w UV. Where I'm going to drain and move the tank I might run the filter on the 75g to seed it a bit. It'll probably be a month before I start thinking of ordering some angelfish.
mentioned, it is osmotic shock that you should worry about, not pH shock. pH shock is a myth ...an old wives tale. A TDS meter (they're cheap) will guide you on this. Try to avoid changes involving more than 50ppm TDS going from one tank to another.
I'm going to have to get an inexpensive TDS meter. Exactly what will I be looking for when reading it? Is it something that I should check frequently, or just occasionally?