You can leave them in the ground, but you'll have to tie up the topgrowth by pulling it straight up into the air, tying with string to keep it there, and then wrap in horticultural fleece. This protects the growing tip from damage during winter. Unwrap and release in Spring.
As for the Callistemon, they're not fully hardy in very cold areas, although a warm, southfacing wall, out of the wind in a very sheltered area would give it the best chance of survival.