Not everyone does the supplements, and you can choose how you want to deal with that. Some people change the routine slightly, but any changes you make should be considered carefully, as you need to respect your penis and make sure you do not overwork it.
Gains are very unpredictable and vary person to person. If I were you, I would just start the routines and see where it takes me.
I hope you enjoy it here. Take your time and focus on penile health. JP90 is not about gaining, it is about conditioning more than anything. The gains you get from it are a bonus, but the routine is meant to ready you for future gains.
Read through the stickies in the beginner`s forums. Welcome to Pegym.com, and I hope you enjoy your stay!