That's really not that important. The brevity will come with some practice, like every other skill you need to fly online well (I hope! I'm working on all this stuff too.)
One of the tricks is doing everything at once, and on time. Poor Aces got a court-martial this weekend for killing a Nato tank. The only thing we could figure was that he'd bombed a bridge a few minutes late, so a tank must have been trying to cross it when he let fly. Not only do you have to not get shot down by migs, sams, or AAA, you have to accomplish your mission on time with some precision too.
Wow the more I read about how the missions evolve and the consequences of getting something wrong the more I want to play online. I better start getting familiar with the cockpit before I know every code.
I wouldn't worry too much about the brevity codes. I printed them out and read them over too, then I get online and play and forget pretty much all of them, heh. When there is a Mig on my tail I pretty much become a jibbering idiot.
I think the brevity codes are something I'll just pick up as I go along. I have already picked up one or two from flying with Jex.
I have been up several times now with the TG guys and let me tell you it is not quite as rigid as you guys seem to think. Yes, we try to use brevity codes other things to try and increase the immersion but our main goal is to have fun with the game, so don't worry if you don't know them.