Just match them by corner. Yes, I would to front first, then when you do the back, do one side, then the other. Of you mismatch diameters on the same corner, the tall one will wear faster, and if you're towing could get too hot.
Valve extenders work just fine as long as you have steel valve stems. Tire stores around here usually mandate steel stems on duallies (proabbly since most end up with extensions).
Best tires I have ever had at Toyo M55 tires. They are a commercial traction tire developed for the NW logging industry. Super good traction and mud, snow, and standing water. Dry traction is nearly as good as the Open Country A/Ts I used to run, and the ride is better. They have stood up well to very primitive rocky-road driving, sometimes at speed. And even towing at capacity at high speeds (80+) through the dessert when its over 110F degrees, the tires never get too hot (unlike the trailer tires).
Main downside is cost. Also, you can hear them hum when the windows are down, they have low speed rating (89 MPH), and they are a heavy tire so it takes a little extra work and weight to balance them. I get about 50,000 out of my rears, and about 80k on the fronts. That is nearly double what I could get out of the Open Country tires.