ok so i only asked this so i could answer it.
the advice on what company monks should keep isn't iirc isn't explicitly said to be a minor precept, one that the effects of not keeping are relatively benign.
but the lotus sutra does say that there is a lot of merit to be found inside it. and of course the greatest sin in buddhism is causing a schism, comparable with killing your mother and father.
however, tripitaka, being modelled on the historical faxiang monk Xuanzang, is an accomplished meditator. so while he may still suffer any bad karma from spending time with monkey, it may be the right thing to do. especially as he is put up to it by the bodhisattva goddess gaunyin.
in the same way that the zen monk nansen cut a cat in half: whether or not he suffered bad karma due to doing so, it was argued, he had to do it to teach his students. eventhough he created suffering
One day a student asked me, 'Does a man of enlightenment fall under
the yoke of causation or not?' I answered, 'No, he does not.' Since
then I have been doomed to undergo five hundred rebirths as a fox. I
beg you now to give the turning word to release me from my life as a
fox. Tell me, does a man of enlightenment fall under the yoke of
causation or not?" Hyakujo answered, "He does not ignore [cloud]
causation [cause and effect]."
so it seems that Tripitaka's actions were deficient because they created suffering, but they were also somewhat skillful, which is what the lotus sutra is all about.
the fact that any act every act, perhaps of any sinfulness, is included within the one vehicle, can be ridden toward perfect enlightenment. it's just that sometimes, we, like tripitaka, act deluded to the truth and end up taking a haphazard course.
perhaps because it is the only one available.