Ah, so all Chan monks receive shikantaza instruction and can practice it whenever they wish?
Monks may receive a lot of instructions or almost nothing, depending on their ordination teacher and the community they live in. There is no such thing as a "Chan monk", there are only monks and nuns. Nowadays in Taiwan the major Buddhist churches have seminars for those who ordain and they study a curriculum. Same goes for Korea and Japan. But a hundred years ago you learnt what and from whoever you could. I don't know what is the situation in mainland China these days regarding monastic education.
Hmm, but didn't Rujing have his own "style" of Chan? If his style wasn't unique then why is it given its own special name (Caodong)?
Caodong is simply a lineage, a virtual system of relationships among elite monastics (primarily abbots). Theoretically the Caodong school's teaching style involved the Five Ranks of Dongshan, and (as mentioned in Fayan's Guidelines for the Zen Schools) "knocking and calling out" (whatever that means), and the five positions of prince and minister (mentioned in the Blue Cliff Record, case 7), and if I recall correctly they also liked to use the Yijing to illustrate teachings. Dogen apparently didn't follow any of that. As for the practice of "shikantaza", on the one hand every Buddhist monk knows sitting meditation, on the other hand the expression itself "just sitting" was most likely created by Dogen himself.