I tried searching if this question has been posted before but I didn't see anything.

I noticed all practice CAT exams have the AWA argument essay first but I did not see the any comment about this in any official guide books.

Does this question always comes first or is it different for everyone?

In my GMAT I had the argument first, but I guess it shouldn't matter. For perspective, I got a 5.5 score and I didn't do one practice essay as I saved my essay skills for the real one. Personally, I find it unuseful to write essays just to practice. My practice was just to look at how GMAT wants essays and then just follow that format and write as much as you can. Not that I got a 6 but really the essay is not a part that we whould waste too much studying. In my opinion!!!

Either can come first on the official test.

I agree that you should spend only minimal time studying the essay -- but what constitutes "minimal time" can vary quite a bit from person to person. The focus of your study for the essay should be: how do I use minimal effort / brain power to get a decent score so that I can save all of my energy for the multiple choice portion of the test? If you're already pretty good at structuring and writing impromptu essays, this may not take much time, but not everyone is naturally skilled at impromptu essays.

Also, it is CRITICAL that you do the essays when taking practice tests. If you don't, your verbal score might tank, as you won't actually be prepared to maintain your stamina for the full length of the test!