In this transcription of an interesting teaching by the 8th Khamtrul Rinpoche, he explains how to go about proceeding on preliminary practices (ngondro), and how its most beneficial to do it with a tradition that one has a karmic connection with. He metions:
The different schools of Buddhism that developed in Tibet, Japan and elsewhere are all teachings of Buddha Shakyamuni. Within the Tibetan traditions there are four main schools—Nyingma, Kagyü, Sakya and Gelug. Do not consider the teachings of a particular school to be more advanced, for all traditions teach tantric meditations that can lead to ultimate realization within one lifetime. However, when commencing the preliminary practices it should be clear which system of teaching is more beneficial to your mind, according to your past karmic connections. By following the tradition most suited to you, you should have faith in the Fully Awakened Being and his Truth without any discrimination, because sectarianism is not only detrimental to your practice but is also an unwholesome action leading to miserable results. If you avoid this error, your practice will flourish and will proceed rapidly through all stages of the path.