There's no reason to think that the Gelugpa approach is normative.
Of course, being prepared to practice the Yidam well, is a very good idea.
What that looks like is very dependent on the qualities of the student, or lack thereof.
At the very least the student must have the empowerment, or in some cases lung --
from a qualified Lama--will suffice. Whether there are any specific practice requirements
given, again, is entirely dependent on the Lama. His or her view of teaching and of the student
I know of several Lamas who almost *never* teach the truth of suffering, simply because it doesn't
function as a motivator for Westerners. They prefer to start with Buddha Nature and inspire students
to practice based on the qualities of the various paths. This is clearly a kind of reverse Lam-Rim!
Renunciation, in that case, is an outcome of choosing to focus on intangible wisdom phenomena
instead of gross, heavy material/worldly phenomena. And Bodhichitta can arise, as can realization of
emptiness, also from practice of the paths. So there may be no prerequisites whatsoever for Yidam.
It is not unheard of for the Three Roots and/or protectors to be practiced concurrently with Ngondro.
Also, what are called preliminaries (ngondro) in the Nyingma and Kagyu are also often understood to
actually be the main practice in those schools.
The main point is, in my estimate, the seriousness with which one practices. Commitment can have
either internal or external locus. But the texts are clear that 2 factors are absolutely necessary:
devotion and samaya.