What is your primary language? You seem to be doing very well with english. To answer your questions. Yes you can add muscle with P90X, it's all about how much you eat. If you are skinny and need to pack on some mass you'll need to eat more. To do this I suggest you subscribe to an application like Myfitnesspal.com and track your calorie intake.
As for the nutrition plan, no you dont need to follow it, most of the people I help don't follow it. You just need to track your calories and adjust your macros accordingly. For you I would suggest 50% carbs 30% protein and 20% healthy fats. Eat what you like as long as its clean and you'll do great!
You can still drink Syntha 6 protein or you can use the P90X Results & Recovery formula, it contains more than just protein and it has the proper mix to help you recover fast and effectively.
You will be fine with your regular chinup bar as P90X doesnt have any neutral grip pullups. In the videos they use a regular straight bar. If you want a P90X bar they sell them on this site or you can get it off ebay. On ebay it will be a knock off and be careful because the quality might not be the same.
If you need any more help feel free to ask, I am here to help you. Feel free to send me a buddy request if you want to chat privately, if not I will continue to answer your questions here.
Have a nice day