so if subrosia is located in the twilight realm, is there some sort of magic protecting hylians who enter it from becoming spirits or link becoming a wolf?

Nope, because there’s really no need for it.  The only reason anyone who isn’t native to the Twilight Realm would succumb to the atmosphere is if they don’t have magic themselves, as in their innate magic. The Zonai (who the Twili were before they were banished to the Twilight Realm) were at risk of dying out because the goddesses stripped them of their magic entirely. They had nothing as punishment for their hubris, but luckily for them the inhabitants of the Twilight Realm, including their gods, took merciful pity on them and allowed them to survive, and eventually adapt to the world.

Even still, you won’t become a spirit entering the Twilight Realm. It isn’t some form of afterlife. If you don’t have magic to help acclimate your body to the different atmosphere you’d grow weaker before eventually succumbing to the Twilight. That only got deadlier when, during the Twilight Princess events, Zant took over the realm and corrupted it, and it’s only through the goddesses’ intervention and Link’s destiny as the Chosen Hero that he escaped death and was transformed into a wolf. A Wolf in specific since wolves are considered sacred animals in the Twilight Realm. That wouldn’t happen to anyone else had they been yeeted into the realm at the time.

Now it’s entirely safe for anyone to enter. If anything, to help Light-realmers’ bodies to acclimate to the Twilight Realm faster, they’d have to eat their food.