During the 15's especially, there is no need for isolations. The entire program can thrive on the ideas behind Bill Starr's 5x5 routine which is that driving the compound exercises upward over time will lead to the gains most are seeking. Isolations can really be saved for those who have basically capped out their mass gains and are looking to eek out whatever they can through pumping fluid into the muscles. That, or for working on sticking points in lift, ala WSB.
I know that if I go some arm isolations for a month or two, my arms swell up a little, but if I stop them, the mass escapes pretty quickly. That never happens, even when I cease rowing and benching due to taking a break from lifting.
The 15's need only three exercises, maybe four. Bench. Squat. Row. Even better would be Pullup, Dip & Squat, but not many can pullup or dip their bodyweight +10-20 lbs.
Those three exercises would cover your entire body and would allow that phase to complete its goal - joint & connective tissue prep for the heavier loads to follow.