With your arms, I usually go with the max weight - it's 2 different muscles you're working. But be sure you're not going to blow it out on the very first set of the very first exercise. You'd like to make it through the entire workout.

If you are supersetting the same muscle, then I would do enough weight to feel a burn and enough weight so that on your next exercise you're not too fatigued to complete the same amount of reps.

As far as scap pinch, you can do it on a row machine, or with some dumbbells. I prefer some sort of row machine so you don't have to bend over. Basically how the movement works is it's kind of like a row, but you don't bend your arms - squeeze your scapulas (shoulder blade) togeter. If you need pictures, let me know.

Time in between sets. Whatever you need. I do whatever it takes to catch my breath. Legs seem like a damn cardio workout, so I might take a little longer break in between sets.