Milarepa tamed them...
oh what's the use.
You are basing this on a biography that was written five hundred years after the fact.
So let me get this straight. The Nyingmpa texts are reliable and the Kagyu texts are not.
Tsang Nyong Heruka's bio of Mila is an inspired piece of historical fiction based on the many oral traditions he collected travelling all over the Himalayan regions where Milarepa wandered. While I have no doubt that Mila adopted the Tsheringma sisters as guardians since the presence of the five Tseringma sisters as guardians in the Kagyu school is well attested, I don't see any reason to accept Tsang Nyon's narratives at face value.
Further, the permission ritual of Tseringma in the Kagyu sNgags mDzod clearly explains how Padmasambhava bound Tseringma under the secret name "Vajrasamantabhadri", and later how in particular Tseringma was made a specific guardian of Kagyu school by Milarepa.
As I read the account given in this permission ritual, Tseringma was testing Milarepa's yogic ability.
Finally, there is no contradiction between stating, for example, "...among non-human disciples, Tseringma was Milarepa's chief student" and understanding that the twelve tenma were Dzogchen guardians from the beginning.
Generally, however, for Dzogchen, among the twelve Tenma, Dorje Yudronma is regarded as more important. For the Kagyus, Tseringma is more important.