I'm planning on visiting Nellis AFB for Red Flag 18-2 and Rainbow Canyon area. I'm not sure what focus length is suitable for these two places? The longest I have is 200mm, and I'm thinking about renting a lens.
Rainbow Canyon is not part of the Nellis Range complex and isn't part of the geographical area used for training during Red Flags. You might be out there a couple of days and see nothing.
I don't know where you plan your vantage point near Nellis, but I would surmise that you need something bigger than a 200mm unless you can get within a 1/4 mile of the runways.
If they are landing on the 3's, you don't need a particularly big lens, as long as you get yourself in the right spot. Forget anything on the ground though with 200mm and if they are using the 21's, you will definitely need a bigger lens and hopefully some cool temperatures to cut down on the heat haze.