Why don't you buy SS, so you can read Rip's stuff advice on this? No offense or anything.. but seriously.
Keep going with linearized gains as long as you can. Subbing is okay *IF* it somehow fits one of your goals. Otherwise, it's better to man up and learn the exercise(s) correctly because the ones he prescribes will give you the most benefits.
There is NO reason for you to worry about periodization unless your squads and deads are above 2-2.5x your bodyweight.