Oh absolutely this is just a guide to get the most realistic sliders for those players who play on All-Madden.. When I play Franchise Mode I like to get realism out of the game rather than putting up massive numbers and beating down teams 100-0, you know? But absolutely if you find that some area is too tough or too easy for you change the slider accordingly and let me know how you do!
And unfortunately, once you're in a game the sliders are locked.