Can Prowler replace the Single Leg/Ab Assistance portion, if so how would I program this?
buy the book… its written inside 
I have all the books besides Forever.
That’s why I asked on this forum.
It depends on the following:
Your complete program (Leader/Anchor).
Your goals.
Your main work/supplemental work.
Your current fitness level.
There is no one answer; rather it has to fit correctly. Understand that hard conditioning needs to be accounted for, and not just in your assistance work. This is especially true if you aren’t in great physical shape.