Doing Assistance Work Between Main/Supplemental Lifts?

I’m currently doing two cycles of the 531 Prep & Fat Loss routine as my Leader as shown in the Forever book and once I’m finished was going to run two Anchor cycles of 531 PR Set with Joker Sets and FSL (5x5) as the Supplemental work as described in the Forever book.

My question is, if my bar speed remains good can I do my Assistance work in between each set of the Main lifts and Supplemental work?

Yes.

I usually set an assistance lift as a primer to my main lift, but aside from that, I superset all assistance with either itself, or the main movement. I cant stand assistance work, and try to get it done as quick as possible.

Yes, this is fine.