Can I combine Athlete 20XX with other training programs?
Nothing stops you from doing that, but we don't recommend this. Athlete 20XX is a stand alone training program and will work amazingly well as long as you give it your full focus.
If you really want to do different styles of training / workouts other than Athlete 20XX, we recommend you do this on rest days or continue doing it after you have mastered the Athlete 20XX program.
Generally speaking, it's better to focus on 1 thing and do it well than to focus on many things at once.