FSL and BBB are two different supplemental lifting schemes. You would do one or the other, but not both.
A typical approach would be to do 2 cycles using BBB as the supplemental approach, then do an “anchor” where you add in the “+” sets on the main 531 lifts, and do FSL as the supplement.
Actually FSL isn’t a supplemental program per se. FSL 5x5 is, for example, and so is FSL AMRAP (although it doesn’t seem to be very popular from what I’ve gathered).
You can do BBB with FSL weights but I’m suspecting, like Wendler states in the book, that it takes a pair of balls.
Kind of semantics. But FSL AMRAP is called a “widowmaker” where you go for 20 reps. I have used these templates, and they’re pretty great.
I run BBB with FSL weight. As long as you pick a legit TM, it’s not a problem to do 5x10 at FSL weight. It’s hard work, but it’s definitely doable. For example, if you can get 5 solid reps with your TM, you can definitely get 5x10 at 60, 70, and 80% of that weight (which corresponds to FSL weights). Heck, if you do Krypteia you are not only doing 5x10 at FSL, but doing weighted pull ups or weighted dips between each set.