It is not necessary to first take the pratimoksa vows but it is usually recommended - the pratimoksha is usually included in the first of the three Bodhisattva moralities.
Agreed as this is also taught in Mahayana to some interpretation but I am also thinking that perhaps, there are some who may not be ready for the extensive scope and practice of the Bodhisattva Vows so this is where the Pratimoksa vows come in as an introductory or foundational scope and practice, hence as you said, 'recommended' and 'necessity'?
The relationship between how the pratimoksa support bodhisattva morality,and whether bodhisattva pratimoksa is unique is explained differently by different masters see for example Asanga, Atisha, Sapan and Tsongkhapa.
Yes I have read that before, especially in terms of karma, effect and motivation amongst the many considerations.
But and it is here that I am intrigued by another matter, related to 'necessity'.
If the Bodhisattva Pratimoksa is so 'complete' and even encompasses the Sravaka Pratimoksa, in the context of monastic ordination, why didn't these masters receive ordination utilising it then, like what the Japanese did in setting aside the Sravaka Vinaya Pratimoksa in favor of the Bodhisattva Pratimoksa?
Why do they still receive monastic ordination under the systems of Mulasarvastivada for Sapan & Tsongkhapa, the Mahasamghika for Atisa and in Asanga's case, some say it could be from Mahisasaka or Mulasarvastivada, all of which are the Sravaka Pratimoksa systems (which today one can see the complexities of ordination processes) whereas the Bodhisattva Vows do not contain monastic vows nor imposes celibacy unlike the Sravaka Pratimoksa for the ordained, hence its openness to all, ordained and laity alike.
And the other matter is also the traditional sevenfold classes of disciples of bhikshu, bhikshuni, sramanera, sramanerika, siksamana, upasaka & upasika are derived from the Sravaka Pratimoksa model according to the level of precepts/vows taken but the Bodhisattva Vows do not have such to the same effect in that all seven classes are holding the same number of vows in the Bodhisattva Pratimoksa system?