Should I follow the P90x nutrition plan?

I'm just starting P90x and I think that the nutrition plan is geared toward fat people. I'm really skinny so I am trying to gain weight and muscle so I'm wondering if I should add more calories. Also i can't afford beachbody's supplements so I was wondering if taking a multivitamin and whey protein instead is a good replacement? BTW I'm doing doubles because I want endurance for sports.