Kale and carrots benefit from a bit of frost, it enhances their flavor.

I wouldn't bother to cover until frost is predicted.

Covering is always a tradeoff against light levels. You want them to get as much sunshine as possible (unfiltered through row cover etc) before the warmth provided by cover becomes more important.

I plan to cover carrots with straw to keep the ground from freezing so I can continue harvesting. Kale doesn't get any cover. Beets can stand up to light frosts.

I would go ahead with hoops and row cover on broccoli, cabbage and cauliflower but throw a blanket over at night if it's going to get really cold. Take off in the morning. They can tolerate some cold. I have a high tunnel and it gets as cold inside as outside at night so I still have to use row covers inside when it gets colder.

I have not used plastic and hoops because I worry about ventilation. If' it's a sunny day, even with double door open and sides rolled up, my high tunnel can get very warm. Many days in summer it got over 100 but peppers loved the extra heat. I debating now when to roll down the sides. A frost is probably going to happen Friday night and I'm going to be away.