Chanting Pancha Tattva mantra during japa
In short, no.
Srila Prabhupada did recommend that we chant the Pancha Tattva mantra:
sri krishna chaitanya
sri advaita, gadadhara
. . .before we chant the Hare Krishna mantra, but he didn't say that the Pancha Tattva mantra must be chanted in between each round of japa. He didn't follow or institutionalize this practice. Many devotees do, however, and there's certainly no harm. But it's not a requirement.
Often we see practices in ISKCON that have become institutionalized without any basis in guru or shastra. It's important to remember that our system is parampara, from master to disciple, and practices that spring up out of popular sentiment or mental concoction—however widespread—are not meant to be perpetuated just because they're popular.