Nope, nope, nope, and if you can, please get your vet to tell you his thought behind that recommendation.
The only thing the PP would do on the heels of Quest is kill the EL3 bugs, and once Quest already killed the other stages, there is NO benefit to going after those, because they would not be a significant issue at that point.
It may also be detrimental to the health of the horse, with the moxidectin being well in the system, and then pumping a double dose of fen for another 5 days. No benefit, potential harm.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET