Hi, and welcome to ATP. Hope we can help with this.
Your bad clay soil is not a good situation for hydrangeas, as they like an acid, loamy soil. They should be hardy in your climate zone in the ground for sure. But in pots they will be much more susceptible to freezing. How big are the giant pots? Are they in the sun during the day in winter? (south or west facing?) This would warm them up daily, and possibly keep them from cooling off enough to freeze at night. Also being close to the wall of the garage could help. Any way you could drag them into the garage on particularly cold nights?
Yes, they will lose their leaves over the winter, but you could plant the pansies in the giant pots so there will be color even though the hydrangeas don't have their leaves. If the hydrangeas survive this winter, there might not be room for pansies next year though.
Take some cuttings of your coleus before it gets bit by the cold, bring them in and root them in a glass of water for new plants in the spring. (just in case the hydrangeas die in the giant pots, it really depends upon how cold the winter is) Coleus make pretty little house plants on a windowsill through the winter.
