Not sure if I am to be considered a veteran. My opinion, I have found that it goes in cycles. I came from quite over weight, so, I generally only do SimpleFit and maybe some other brief workouts certain other days. I have tried MH Spartacus work out, kettlebells and some other home made (usually some sort of bodyweight) training. But, I have mainly tried to improve my time, reps etc and level up exclusively in SimpleFit. I have experimented on the D2 workouts and rest weeks, changing hand positions in pull ups and push ups, adding weight and grouping the sets. If I do add stuff in, I generally dont do anything before D1 or D3 as these are the days I rely on to guage performance. Usually only adding stuff in after D1 or D3. There are a lot of people on marksdailyapple.com who think it is impossible to overtrain. I am not so sure I agree as I ached badly after last weeks D1 (most ever rounds on L4).
The Simplest advice is to find what you enjoy and be careful, adding it in a bit at a time. The last thing I want to do is hurt myself lifting and stop myself from doing SimpleFit which has been my fitness saviour!