The book says that assistance is whatever you want it to be - so 5s Pro falls under that category.
I've started a new round of the "28 weeks of training" (my third in as many years) and on my second cycle right now. I'm doing BBB as my assistance work after the main work for all cycles, like on Squat day I'm doing BBB dead lifts. I've also "thrown in some curls and triceps" work toward the end of my workout on upper body days and abs, hypers on lower body days; but it's not even close to my focus. Just be sure to superset your assistance template with "some sort of tugging." With dead lifts and squats I'm always doing either band pull aparts or chins, same with upper body; even during the main work I've got a band with me and do pull aparts, face pulls, etc...
Depending on mood, how I'm feeling and how much time I have dictates the "do I just call it a day." I always do my 5/3/1 sets and reps and everything else is secondary. As Jim says, and I paraphrase:
"if you push the main work it will not matter how you do curls and face pulls."
Just don't try to do the 28 weeks of training and an accessory template without a good recovery protocol. You must eat like it's your second job and rest like a rock. You also need to condition appropriately - I walk 2 miles a day at least 3 days a week and run hills twice a week.
One rule I give myself is if I can't get my workout done under 60 minutes, I've programmed too much. A sample day of mine right now is this which I can get done in under an hour:
Squat 5/3/1 rep maxes + LSF AMRAP (super set w/ band pull aparts)
Dead Lift BBB 5x10 @ 50% (superset with chins or band pull aparts) - I don't rest much between these sets at all for any of the days, 30-60 seconds tops, typically I do a set of 10 deads, then do 20 pull aparts then do another set of 10, etc...
Lying leg curl 3-5x10
Hyperextensions 2 drop sets, 1 straight set (ex drop set: 50# x amrap, 25# amrap, 0# amrap)
ab wheel 3-5 sets of 10-20 reps
edit: I don't do rest pauses on lower body day, but doing rest pauses on upper body days for the LSF AMRAP)